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EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH
MLSS PROVIDED WITH A SPEECE CONE AERATION SYSTEM
AS AN ALTERNATIVE FOR SANITATION PROVISION IN
EMERGENCIES
Carlos Mauricio Barreto Carvajal
MSc Thesis UWS-SE-2015-15
EVALUATION OF A PILOT MBR SYSTEM OPERATED AT
HIGH MLSS PROVIDED WITH A SPEECE CONE AERATION
SYSTEM AS AN ALTERNATIVE FOR SANITATION
PROVISION IN EMERGENCIES
Master of Science Thesis
by
Carlos Mauricio Barreto Carvajal
Author
Supervisors
Prof. Damir Brdjdanovic, PhD, MSc (UNESCO-IHE)
Mentors
Hector García, PhD, MSc (UNESCO-IHE) Tineke Hooijmans, PhD, MSc (UNESCO-IHE)
Examination committee
Prof. Damir Brdjdanovic, PhD, MSc (UNESCO-IHE) Hector García, PhD, MSc (UNESCO-IHE) Aridaí Herrera, MSc (JCI Industries, Inc. - Ext)
This research is done for the partial fulfilment of requirements for the Master of Science degree at the UNESCO-IHE Institute for Water Education, Delft, the Netherlands
Although the author and UNESCO-IHE Institute for Water Education have made every effort to ensure that the information in this thesis was correct at press time, the author and UNESCO-IHE do not assume and hereby disclaim any liability to any party for any loss, damage, or disruption caused by errors or omissions, whether such errors or omissions result from negligence, accident, or any other cause.
©2015 by Author.
This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.
i
Abstract
A size (volume) reduction for membrane bioreactor systems (MBR)s is constrained by the
maximum amount of biomass that can be accommodated in the aerobic basin. The maximum
achievable biomass concentration in the aerobic basin, known as the mixed liquor suspended
solids (MLSS), is limited mainly by the extremely low oxygen transfer efficiency observed at
MLSS concentrations higher than 20 g/L. Another limitation for achieving such high MLSS
concentration in MBRs is the membrane permeability; causing limitation of the performance
by membrane fouling. A pilot MBR system with a hydraulic treatment capacity of
approximately 1 m³/d was located at the Harnaschpolder wastewater treatment plant in Delft,
The Netherlands. The pilot MBR was operated at high MLSS concentrations (up to
approximately 25 g/L). The MBR system was provided with an alternative source of oxygen
supply (Speece cone system) to overcome the oxygen transfer limitations exhibited by
conventional bubble diffuser systems. The MBR system showed very high COD removal
efficiencies (>90%) even at hydraulic retention times (HRT) as low as 3.4 hours. The MBR
performance was evaluated by monitoring the influent and effluent water quality, the
membrane permeability, the sludge filterability, and the oxygen uptake rate (OUR).
The Speece cone aeration system proved to be highly efficient in delivering dissolved oxygen
at the evaluated MLSS. OUR values greater than 100 mg/L/h (at 14 g/L MLSS) were
reported and even higher than 150 mg/L/h at MLSS above 20 g/L.
The membrane permeability decreased as the MLSS increased; however, improvement in
permeability values was observed between 10 to 14 g/L MLSS. Sludge filterability values in
the range of "poor filterability" were observed; however, a reduction in the filtration
resistance (or a filterability improvement) around the 10 g/L MLSS was noticed.
iii
Acknowledgements
This research work is dedicated to my family for their never ending support; each one of you
made this possible in one way or another. Mom and Mimi, my personal counselors and Dad
how I wish you were here.
I would like to thanks very much OVIVO and the Bill and Melinda Gates foundation for
providing funds to support my studies in The Netherlands.
I also want to thank my mentors Hector and Tineke for being there when I needed them and
my supervisor Damir Brdanovic for always trusting on me.
UNESCO IHE family, thanks for making me feel I was at home all the time, you guys always
do a terrific work, with a special mention to my laboratory staff friends for taking every
request I made no matter what.
I would also like to extend my gratitude to ECO2 for the technical and financial support.
Han and Justina from Evides at the Harnaschpolder wastewater treatment plant, thanks for all
of your patience and help.
v
Table of Contents
Abstract i
Acknowledgements iii
List of Figures vii
List of Tables ix
Abbreviations x
1. Introduction 12
1.1. Background 12
1.2. Problem Statement 12
1.3. Research aim and Objective 13
1.4. Research questions 14
2. Literature Review 15
2.1. Conventional Activated Sludge 15
2.1.1. Key Process Aspects 17
2.1.2. MLSS and Sludge Volumetric Index (SVI) 18
2.2. MBR technology 18
2.2.1. Process key aspects 20
2.3. CONVENTIONAL AERATION METHODS 25
2.3.1. Factors affecting oxygen transfer 26
THE SPEECE CONE 28
3. Materials and Methods 30
3.1. Worksite description 30
3.1.1. Harnaschpolder Wastewater treatment plant 30
3.1.2. Pilot Hall 32
3.2. Equipment description 32
3.2.1. Pilot MBR 32
3.2.2. Speece cone setup 35
3.2.3. The Delft Characterization Method Installation (DFCi) 37
3.2.4. Other sub-systems 39
3.3. Analytical methods 42
4. Results and Discussion 45
4.1. Startup process 45
4.1.1. Operational conditions 56
4.2. MBR Performance 59
4.2.1. Water Quality 59
4.2.2. Treatment capacity 62
4.3. Activated Sludge properties 73
4.3.1. Mixed Liquor Suspended Solids concentration 73
vi
4.4. Oxygen Dynamics 80
4.4.1. Oxygen Uptake Rate (OUR) 80
4.4.2. Comparison with conventional systems 86
4.4.3. Oxygen transfer using the speece Cone aeration system 88
4.5. Full scale application feasibility 92
4.5.1. Supporting biomass growth 92
4.5.2. Using convention aeration systems or concentrated oxygen delivery systems 92
5. Conclusions 93
References 95
vii
List of Figures
Figure 2.1 Simplified process diagram of the conventional activated sludge (CAS) treatment system.
(Geilvoet, 2010). ... 16
Figure 2.2 MBR sidestream configuration. (Geilvoet, 2010) ... 19
Figure 2.3 MBR immersed configuration. (Geilvoet, 2010) ... 20
Figure 2.4 Time to filter (TTF) method to determine filterability.(Geilvoet, 2010) ... 21
Figure 2.5 Sludge filtration Index (SFI) to determine filterability. (Geilvoet, 2010) ... 22
Figure 2.6 Schematic of fouling rates in long term operation of full scale MBRs.(Drews, 2010)... 23
Figure 2.7 Factors affecting submerged membranes fouling. (Le-Clech et al., 2006) ... 23
Figure 2.8 Inter relationships between MBR parameters and fouling. (Judd, 2010) ... 24
Figure 2.9 Simplified scheme of membrane cleaning methods. (Henze, 2008) ... 25
Figure 2.10 Disc and tubular diffusers for fine bubble aeration. Stanford Scientific International LLC. .. 25
Figure 2.11 MLSS concentration and alpha factor correlation. (Ando, 2013) ... 27
Figure 2.12 Oxygen transfer from gas phase to cell or solid particle. Adapted from (Garcia-Ochoa & Gomez, 2009) ... 27
Figure 2.13 Speece cone. SOURCE: ECO2 TECH website 2014 ... 28
Figure 2.14 KLa and SOTE increase with higher discharge velocities. (Ashley et al., 2008) ... 29
Figure 3.1 Influent and effluent water quality at Harnaschpolder WWTP ... 31
Figure 3.2 General view of the secondary settlers HNP WWTP. ... 31
Figure 3.3 Harnaschpolder WWTP pilot hall view ... 32
Figure 3.4 MBR Membrane module ... 33
Figure 3.5 Low pressure blower for conventional aeration ... 34
Figure 3.6 MBR control box. ... 34
Figure 3.7 Electromagnetic flow indicator transmitter (FIT) ... 35
Figure 3.8 Variable frequency drive to control the sludge pump discharge. ... 35
Figure 3.9 Positive displacement pump for the sludge closed circuit. It had to be assembled on site. .... 36
Figure 3.10 Speece cone before and after installation ... 36
Figure 3.11 Equipment layout schematic ... 37
Figure 3.12 The Delft Filtration Characterization installation at the TU Delft. ... 38
Figure 3.13 The DFCi at the Harnaschpolder WWTP (left). DFCi process diagram (Lousada-Ferreira, 2011) ... 38
Figure 3.14 Gas feed system schematic for design and purchasing purposes ... 39
Figure 3.15 Oxygen gas feed system components ... 40
Figure 3.16 Alternative carbon source feed system ... 40
Figure 3.17 Submersible pump in the sludge basin connected to the sludge line (Up left); Sludge line at the pilot hall connected to a reinforced hose (Up right); Reinforced hose and MBR reactor (down left); Activated sludge discharge in the MBR reactor (down right). ... 41
Figure 3.18 DFCi output file example with the added resistance against the specific permeate production. ... 42
Figure 4.1 Electric integrity check performed by a external specialized company. ... 46
Figure 4.2 Broken manifold (Up) and new manifold for membrane scouring (Down) ... 47
Figure 4.3 Taking the membrane module out of the reactor using a forklift. ... 48
Figure 4.4 First attempt to repair the broken membrane module ... 48
Figure 4.5 Second attempt to repair the membrane module. ... 49
Figure 4.6 Temporary modification for SBR operation mode. Floating decanter (Up left); Reactor in filling stage (Up right); Reactor in decanting stage (Dow left); Control box modification (Down right). ... 50
Figure 4.7 Original flow meter filled with activated sludge (Left); New flow meter after installation. .. 51
viii
Figure 4.9 Second level switch installed (floating type) ... 52
Figure 4.10 Third level switch installed... 52
Figure 4.11 Damaged stator from sludge pump. ... 53
Figure 4.12 Greasy slime appearing in the permeate tank when the growth and permeability decreased . 54 Figure 4.13 Membrane module chemically assisted cleaning process and sludge inoculation ... 55
Figure 4.14 Operational conditions overview ... 57
Figure 4.15 Operational conditions overview and unplanned stops ... 58
Figure 4.16 Influent and effluent COD concentrations and removal efficiency for different MLSS concentrations ... 59
Figure 4.17 Calculated MBR MLSS concentration as a function of HRT. (Yoon et al., 2004) ... 60
Figure 4.18 Net MBR yearly cost reduction for aeration and sludge treatment at higher MLSS concentration. (Yoon et al., 2004) ... 61
Figure 4.19 HRT and COD removal efficiency. ... 61
Figure 4.20 MBR Operational pressure profile for 20 cycles (mbar) ... 62
Figure 4.21 Pressure values example (max, min, mode) for each operational cycle (zoom in from Figure 4.20). ... 63
Figure 4.22 TMP results at different MLSS concentrations ... 63
Figure 4.23 Specific flux (LMH/applied pressure) ... 64
Figure 4.24 TMP at different MLSS concentrations for a fifty cycles period. ... 65
Figure 4.25 Net flux for increasing MLSS concentration. (max, min and mode values) ... 65
Figure 4.26 Flux for increasing MLSS concentrations (upper concentration range) during a fifty cycles period. ... 66
Figure 4.27 Normalized Operational Permeability (OPn) for increasing MLSS concentrations. ... 67
Figure 4.28 Normalized Operational Permeability (OPn) for a fifty cycles period. ... 67
Figure 4.29 Flux, TMP and Permeability for different MLSS concentrations (7.6, 8.5, 10.32, and 10.36 g/L) ... 68
Figure 4.30 Flux, TMP and Permeability for different MLSS concentrations (12.21, 12.7, 14.54 and 15.41 g/L) ... 69
Figure 4.31 Flux, TMP and Permeability for different MLSS concentrations (18.7, 23.96, 23.3 and 22.79 g/L) ... 71
Figure 4.32 Foam production after dosing sodium acetate and changes on sludge characteristics. ... 72
Figure 4.33 Slimy substance in the permeate tank after the sodium acetate dosing. ... 72
Figure 4.34 Sealed vessels used to reduce the aerobic tank volume ... 73
Figure 4.35 Observed MLSS concentration (target and measured) and operational parameters for each point. ... 75
Figure 4.36 MBR activated sludge. Low MLSS (left). High MLSS (right) (Lousada-Ferreira, 2011) ... 76
Figure 4.37 Measured Filterability values ... 77
Figure 4.38 Resistance (R), Flux (J) and Pressure (TMP). Clockwise from top left: ID#5 (7.6g/L), ID#6 (8.57 g/L), ID#7 (10.36 g/L), and ID#8 (14.54 g/L) ... 77
Figure 4.39 Resistance (R), Flux (J) and Pressure (TMP). Clockwise from top left: ID#10 (12.21 g/L), ID#12 (18.7 g/L), ID#15 (23.79 g/L) ... 78
Figure 4.40 Added resistance. Clockwise from top left: ID#5 (7.6g/L), ID#6 (8.57 g/L), ID#7 (10.36 g/L), ID#8 (14.54 g/L) ... 79
Figure 4.41 Added resistance. Clockwise from top left: ID#10 (12.21 g/L), ID#12 (18.7 g/L), ID#15 (23.79 g/L) ... 80
Figure 4.42 Calculated and measured Oxygen uptake rate (OUR20) ... 84
Figure 4.43 Specific Oxygen uptake rate (SOUR20) ... 85
Figure 4.44 Benefits of high MLSS MBR in terms of volume and HRT reduction. ... 87
Figure 4.45 Required volume and OUR for different influent COD concentrations ... 88
ix
List of Tables
Table 2.1 WWTP average treatment values and discharge standards in the Netherlands. Modified
from (Geilvoet, 2010). ... 17
Table 2.2 Average treatment values and efficiency removals in Nordkanal wastewater treatment works in Kaarst, Germany. (Henze, 2008). ... 19
Table 3.1 Analytical methods materials and ranges ... 44
Table 4.1 Operational conditions overview and COD values ... 57
Table 4.2 MBR possible operational states ... 62
Table 4.3 TSS target and measured MLSS concentration for each MBR operational point ... 74
Table 4.4 Measured Filterability values ... 76
Table 4.5 MBR Operational conditions and OUR values ... 81
Table 4.6 Required volume comparison between conventional systems and high MLSS MBR ... 86
Table 4.7 MBR and Speece cone set up operational conditions ... 90
x
Abbreviations
BOD :
Biochemical
oxygen
demand
CAS :
Conventional Activated sludge
COD :
Chemical
oxygen
demand
DFCi :
Delft
Filtration Characterization installation
DFCm
:
Delft
Filtration Characterization method
HNP :
Harnaschpolder
HRT :
Hydraulic
retention
time
LMH :
Liters/m²/hour
MBR :
Membrane
bioreactor
MLSS
:
Mixed
liquor
suspended
solids
OPn :
Operational
permeability
(normalized)
OUR :
Oxygen
uptake
rate
SDOX
:
Supersaturated dissolved oxygen
SOUR
:
Specific
oxygen
uptake
rate
SRT :
Sludge
retention
time
TMP
: Transmembrane pressure
TSS :
Total
suspended
solids
UBOD
:
Ultimate
BOD
EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE
EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE
AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 12 This chapter contains background information about the topic of concern in this research, the problem statement, the objectives, and the research questions.
1.1. Background
The operation of membrane bioreactors (MBR)s at high mixed liquor suspended solids (MLSS) concentrations (higher than 20 g/L) may introduce advantages in terms of footprint and volume reduction. However, the maximum achievable MLSS concentration is currently limited due to the extremely low oxygen transfer efficiency observed when using conventional aeration methods such as fine and coarse bubble diffusers.
Alternatives sources for providing dissolved oxygen in MBRs operated at high MLSS have been researched including the use of pressurized supersaturated oxygenation devices fed with pure oxygen rather than compressed air (SDOX system by BlueInGreen). Bilal, (2013) and Makgatha, (2014) operated a supersaturated dissolved oxygen delivery unit (SDOX system) manufactured by BlueInGreen at laboratory conditions. Both authors reported excellent oxygen transfer capabilities with lower alpha factor reduction effects compared to conventional bubble diffusers when conducting experiments at high MLSS concentrations (as high as 35 g/L). In addition, Makgatha, (2014) reported oxygen uptake rates (OUR) values of up to 300 mg/L-hr when operating a laboratory scale batch reactor aerated with the SDOX system at MLSS of approximately 20 g/L.
1.2. Problem Statement
Reducing footprint requirements (together with operational costs) is highly desirable for advancing on the decentralization of sanitation provision. In addition, the development of compact wastewater treatment systems (movable and easy-to-transport) may provide an excellent alternative for the provision of sanitation services in applications such as emergency situations (i.e., the provision of sanitation services in emergency camps after the occurrence of a natural or human made disaster). Considering all the existent alternatives for the provision of sanitation services, MBRs present a suitable option with some additional advantages including: the production of a high quality effluent suitable for water reuse, the reliability of the technology, and the operational flexibility to adjust to changes in loads.
The achievement of a further volume reduction in MBRs is mainly constrained by the maximum attainable MLSS that can be sustain in the aerobic basin of an MBR system. The MLSS will determine the required reactor volume. MBR systems are designed to be operated at MLSS values of approximately 8-12 g/L.
CHAPTER 1
EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE
AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 13 Operating an MBR at a high MLSS value (higher than 18-20 g/L) reduces the process required volume and reactor footprint. However, MBRs operated at such conditions will be limited in terms of oxygen demand and oxygen transfer. As the MLSS concentration increases, the oxygen transfer efficiency decreases when using conventional aeration systems such as fine and coarse bubble diffusers. This effect is more noticeable when working at MLSS of 20 g/L or higher, eventually reaching an MLSS concentration where no oxygen transfer can be achieved (Henze, 2008).
The current limitations on MBRs may be uncapped by incorporating innovative aeration systems able to efficiently deliver the process oxygen demand when working at high MLSS concentrations. By incorporating such innovative aeration technology, more compact MBRs may be designed. In addition, existing facilities may be easily upgraded treating more flow utilizing the existent reactor volume. Moreover, the footprint requirement of movable (package) systems for quick response can be reduced. Package MBRs can be used under emergency conditions or in refugee camps where a sudden increase in population (and in wastewater generation) can be treated by adjusting the MLSS set point in the aeration basin. In addition, operating at such high concentrated MLSS values may reduce the related oxygen supply costs considering the high oxygen transfer efficiency of the supersaturated oxygenation systems.
Operating an MBR at such high MLSS concentrations may also have a negative impact on the membrane performance. Therefore, the filterability of that highly concentrated sludge should be further evaluated. A high negative impact on the filterability may considerable affect the capital and operational costs associated with permeate production.
1.3. Research aim and Objective
This research aims at evaluating the performance of a pilot scale 1 m³/d MBR system. The MBR will be provided with an innovative supersaturated oxygen delivery system known as the Speece cone (Speece, 1972). The MBR-Speece cone system will be fed with municipal wastewater. Additional provisions are considered for incorporating an extra source of COD to reach higher organic loads than conventional municipal wastewater organic loads. The pilot MBR-Speece cone system was be located at the Harnaschpolder wastewater treatment plant in Delft, The Netherlands. The innovative aeration system (Speece cone) has been successfully used for lakes and ponds remediation in the US (Ashley, Mavinic, & Hall, 2008) reaching dissolved oxygen super saturation conditions of up to 300%. Another key parameter influencing the feasibility of the MBR operated at high MLSS values is the sludge filterability. The sludge filterability will impact the maximum allowable trans-membrane pressure (TMP) for the permeate production, as well as the required level of maintenance (membrane cleaning) due to the excessive fouling potential that might occur at such high MLSS concentrations. Filterability will be assessed by using the Delft filtration characterization method (DFCm) developed at the Technical University in Delft by Geilvoet, (2010).
The implications of a positive evaluation of the MBR-Speece cone system operated at high MLSS values may include: (i) the possibility for increasing the installed capacity of existing wastewater treatment facilities; (ii) significant footprint reductions on wastewater treatment systems; and (iii) potential for aeration cost reduction.
General Objective
The main objective of this research is twofold:
(i) To set up a pilot scale MBR system provided with an innovative technology for concentrated oxygen delivery (speece cone system)
EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE
AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 14 Specific Objectives:
i. To set up a pilot 1 m3/d MBR in the research facilities at the Harnaschpolder wastewater treatment plant (Delft, The Netherlands).
ii. To reach steady state operational conditions in the MBR fed with municipal screened influent working at a MLSS concentration of approximately 10g/L. At this stage the MBR is operated with the conventional aeration system (fine and coarse bubble diffuser equipment). The baseline conditions regarding water and sludge characteristics will be established including treatment performance (influent vs. effluent water quality), oxygen uptake rate (OUR), filterability, and permeability.
iii. To operate the pilot system at high MLSS values and at high organic loads. At this point the MBR
is operated with the innovative Speece cone aeration system using pure oxygen. The water and sludge characteristics for these specific operational conditions will be evaluated.
1.4. Research questions
The general and specific objectives previously described lead to the following general research question: What would be the performance of a pilot MBR operated at high MLSS concentrations fed with concentrated wastewater using a conventional and an innovative oxygen supply systems?
Specific research questions:
i. Which are the main concerns when installing and running a MBR pilot setup under the previously described specific conditions?
ii. How does the membrane permeability change as the MLSS concentration in the reactor increases? iii. Is the water quality affected by the different operational set points?
EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE
AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 15 This chapter describes the characteristics of both Conventional Activated Sludge (CAS) processes and Membrane Biological Reactor (MBR) systems. Relevant design and operational parameters for both systems are discussed including sludge retention time (SRT), hydraulic retention time (HRT), permeability, filterability, and fouling mechanisms. This chapter also contains information about conventional aeration systems and innovative supersaturated oxygen delivery systems such as the Speece cone technology.
2.1. Conventional Activated Sludge
It was 1913 in Manchester sewage works England, when Adern and Lockett observed how wastewater treatment could be performed faster and with better results when using aeration and sludge recirculation in what they named the activated sludge process (Geilvoet, 2010).
The Conventional Activated Sludge (CAS) process relies on the ability of many different bacteria groups to use most of the substances present in wastewater as an energy source to run their metabolism. To do so, they require a series of conditions to be present that allow growth with the subsequent conversion of the pollutant load into more "ready to work" biomass, the so called activated sludge, and carbon dioxide. Besides environmental conditions such as pH (6.5 to 9), temperature within the mesophilic range (up to 40 ºC), substrate and nutrient availability, the bacteria involved in the process use oxygen as an electron acceptor during the chemical oxygen demand (COD) breakdown. Even though the previously mentioned factors (among other factors) affect the rates at which the metabolic processes occur, it is the oxygen transfer to the active biomass the process limiting the amount of bacteria that can be accommodated in a given volume of an aerobic reactor.
The content of the aerobic reactor (the mixture between wastewater and activated sludge) is commonly called the mixed liquor. The amount of microorganisms per unit of volume is frequently expressed as a concentration of suspended solids (including live biomass and inert material) in the mixed liquor; that is, the mixed liquor suspended solids (MLSS) expressed generally as (g/L) (Henze, 2008).
The CAS configuration consists of an aerated biological reactor where wastewater and activated sludge interact to form the mixed liquor. The reactor is aerated to provide the required oxygen demand. There are many aeration methods and devices to supply dissolved oxygen; fine bubble aeration difussers coupled with low pressure blowers is the most widely used aeration alternative. Surface aerators are also an alternative to deliver dissolved oxygen. Energy costs related to aeration in aerobic treatment systems are of a high relevance in terms of operational expenses (OPEX).
CHAPTER 2
EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE
AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 16 After the biological reactor, the mixed liquor enters the clarifier (or settler) unit where the flow velocity is reduced to separate by differences in density the solid phase (activated sludge) from the liquid phase (clarified water). The clarified water leaves the settler at a controlled flow rate that is calculated using hydraulic principles such as the hydraulic and solids loading (Henze, 2008). While decantation occurs, the activated sludge settles down to the bottom of the clarifier, where it is collected in a hopper and from there it is taken as two different streams:
The waste activated sludge (WAS): is the excess sludge that needs to be discarded in order to keep the reactor MLSS constant. The amount of WAS depends on both the target MLSS, as well as on the designed sludge age or sludge retention time (SRT).
(2.1)
The return activated sludge (RAS): is the rate at which the activated sludge must be returned into the aerobic reactor in order to reach the design SRT. It is usually set at 0.25 to 3 times the design flow rate (Henze, 2008).
A general process scheme is shown in Figure 2.1 Simplified process diagram of the conventional activated sludge (CAS) treatment system. (Geilvoet, 2010). Other activated sludge process configurations may include different recycle streams, and anaerobic and anoxic chambers to allow nitrogen and phosphorous removal as well.
Figure 2.1 Simplified process diagram of the conventional activated sludge (CAS) treatment system. (Geilvoet, 2010).
EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE
AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 17 Table 2.1 WWTP average treatment values and discharge standards in the Netherlands. Modified from
(Geilvoet, 2010).
2.1.1. Key Process Aspects
Hydraulic Retention Time (HRT)
The HRT is defined as the average time water spends in the aerobic reactor; it can be calculated by dividing the tank volume by the influent flow rate. There are two types of HRT, depending on the flow that is used to calculate that parameter:
Nominal retention time: is the purely theoretical time it will take to a water molecule to leave the reactor when it is working at the design flow rate:
(2.2)
Actual retention time: when other return flow streams (RAS, nitrified wastewater) are added up to the nominal flow rate, the same calculation will give the actual retention time which is the real time water spends in the reactor under the specific flow rate configuration:
(2.3)
Modified from (Henze, 2008). Nutrients availability
Bacterial growth will occur whenever the environmental conditions are appropriate and the required chemical compounds are available for microorganisms to build up new biomass. As in any other chemical reaction, bacterial growth (a series of biochemical reactions) stops when the chemical reaction runs out of one the required elements (reagents). Both the oxygen concentrations, as well as the substrate availability fall in this category of required elements (reagents). In addition, nutrients (mainly nitrogen and phosphorous) are also essential reagents to carry out these processes.
Domestic wastewater usually contains enough nutrients to promote bacterial growth in CAS process. The amount of nitrogen and phosphorous need to be in the appropriate ratio with respect to the amount of biodegradable carbon; the general ratio for aerobic treatment are 100:5:1 (C:N:P) expressed as mass per day (Metcalf, Eddy, & Tchobanoglous, 1972). For industrial wastewater this ratio can be different, and nutrients addition might be required to support the biological process (bacterial growth) depending on the wastewater characteristics and the type of industrial process generating that wastewater.
EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE
AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 18 Sludge Retention Time (SRT)
It is the main and most relevant parameter for CAS design and operation control. Also known as sludge age, it represents the average time that solids (activated sludge) stay in the aerobic reactor. SRT defines not only the size of the reactor, but also the oxygen requirements, the amount of WAS and the removal efficiency of contaminants - especially the biological nitrogen removal processes (Geilvoet, 2010). During the design process of a CAS, a SRT value is selected to fulfill the treatment demands at a given MLSS concentration; from there, the WAS can be calculated following as in equation (2.1).
Since the SRT describes the amount of biomass present in the system, the treatment efficiency will strongly depend on it. It is a numeric representation of the treatment "capacity" in a given system and conditions. The required SRT for carbon and nitrogen removal differ since different bacteria groups have different specific growth rates; therefore, they require longer or shorter SRT periods to sustain a certain amount of biomass in the system. In addition, the specific growth rates depend on the temperature. For carbon removal, usually 3 to 5 days SRT is enough; however, for nitrogen removal SRTs range from 3 to 18 days and even longer if the operation temperature is too low, or in other words nitrifying bacteria need more time to breed (Metcalf et al., 1972).
2.1.2. Sludge Volumetric Index (SVI)
The SVI is an empiric parameter representing the volume occupied per sludge mass after a given settling period, it means an indication of the settling ability (or settleability) of the sludge. A good sludge settleability is crucial in CAS system since it influences the secondary clarifier performance and the effluent water quality.
The SVI determination method uses an Imhoff cone to measure the settled sludge volume of a 1 liter mixed liquor sample after 30 minutes of sedimentation (the settleability test). The volume is related to the MLSS concentration to calculate the specific volume in mL/g (Geilvoet, 2010) as below.
(2.4)
2.2.
MBR technology
Membrane bioreactors use a physical barrier (the membrane) to separate the treated water (the permeate) from the mixed liquor instead of a settling unit. The use of a membrane allows reaching better water quality in the effluent. The non settleable material that would escape from a secondary settler in a CAS system will be retained by the membrane. The efficiency of the phase separation will no longer rely only on hydraulic principles and the settling properties of the sludge, but instead will be dictated, among others, by the pore size of the filtration element (Geilvoet, 2010).
EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE
AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 19 Table 2.2 Average treatment values and efficiency removals in Nordkanal wastewater treatment works
in Kaarst, Germany. (Henze, 2008).
In order to separate the treated water from biomass polymeric and/or ceramic (depending on the type of wastewater) ultra filtration (UF) or micro filtration (MF) membranes are commonly used. Two different arrangements can be used: sidestream and immersed filtration. A typical sidestream and immersed membrane arrangement are showed in Figure 2.2 and Figure 2.3 respectively.
Figure 2.2 MBR sidestream configuration. (Geilvoet, 2010)
The main difference between these two arrangements (in addition to the location of the membranes inside or outside the biologic reactor), is the way the different configurations deal with fouling prevention. Sidestream filtration relies on the liquid crossflow velocity to drag particles sitting on top of the membrane creating enough shear force velocities between 1 to 6 m/s (Evenblij, 2006). That implies an enormous energy input for pumping the flow through the membranes than can be 10 times higher than the required for immersed filtration arrangement (Geilvoet, 2010). On the other hand, immersed filtration uses air scouring systems which require less energy, but which are less effective in terms of the achievable shear force applied on the membrane surface; therefore, lower fluxes are applied in this configuration (from 20 to 40 L/m²-h) when compared to the sidestream configuration (from 50 to 100 L/m²-h) (Geilvoet, 2010).
EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE
AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 20 Figure 2.3 MBR immersed configuration. (Geilvoet, 2010)
Hollow fiber and flat plate are the two most common types of membranes used on the immersed configuration. They can operate in a low pressure range (from 0.2 to 0.5 bar) considering the lower energy requirement of the immersed configuration compared to the sidestream configuration with TMP values from 2 to 6 bar (Geilvoet, 2010).
2.2.1. Process key aspects
MLSS
The concentration of TSS in the mixed liquor determines not only the reactor volume but also the load treatment capacity, the maximum allowable flux, and the oxygen requirements according to (Henze, 2008). Together with the SRT, the concentration of TSS is a critical design parameter usually dictated by the strength of the influent wastewater, or in other words, by the capacity of the available substrate to maintain such concentration of bacteria.
The MLSS and the specific sludge properties, depending on the characteristics of the wastewater, will determine the fouling potential which affects directly the permeability, durability, and maintenance requirements for the membranes.
Trans membrane pressure (TMP)
The pressure drop across the membrane or in other words, the required pressure to produce a certain flux (L/m²-h) is referred as the TMP (L/m²-h-bar) as described in Equation (2.5). Considering that the fouling occurs while controlling the system to operate at a constant flux, the TMP will rise to compensate the higher resistance to filtration in the upstream side of the membrane.
L/m²/ h bar
(2.5)
Where:
TMP : Trans membrane pressure (bar) J : flux (L/m²-h)
EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE
AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 21 Permeability
Measuring permeability values is the most popular method to both monitor membrane operation, and control cleaning cycles. Measuring permeability is relatively easy to conduct on site; however, by measuring permeability it is not possible to describe the fouling processes causing the decrease in permeate production. Permeability can be used to describe the current status in a system (the symptom). When measured continuously, permeability can be plotted to evaluate the rate at which permeate production is reduced.
Permeability (P) is defined as the specific flow rate (per unit area) per unit pressure, or the flux divided by the trans membrane pressure (TMP) as it is shown in Equation (2.6) (Judd, 2010).
.
(2.6)
Filterability
Filterability is a sludge property that refers to the fouling potential of an activated sludge. The method determines the resistance that the sludge opposes to be filtered under specific operational conditions (depending on the method used to measure it). The filterability determination has been the focus of attention for many researchers in the last years. As a consequence, new methods for measuring filterability have been recently developed mainly in Europe.
Some of the most popular methods to determine filterability are:
Time to filter (TTF): TTF measures the time it takes to produce 100 ml of permeate though a Whatman Nº1 paper filter using a vacuum pressure of 51 kPa. (Awwa, 1998). The method is shown in Figure 2.4.
Figure 2.4 Time to filter (TTF) method to determine filterability (Geilvoet, 2010)
EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE
AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 22 Figure 2.5 Sludge filtration Index (SFI) to determine filterability. (Geilvoet, 2010)
The Delft Filtration Characterization method (DFCm): DFCm represents the state-of-the-art method to determine filterability. This method is used in this research. The method consists of a complex set-up with an UF single filtration element operated at very specific flux and TMP conditions. The DFCm method is described extensively in the materials and methods section of this thesis.
Filterability allows monitoring how the permeability is affected by the sludge characteristics. The filterability determination may help to identify new methods to prevent reversible fouling. In addition, more efficient filtration systems can be developed for sludge exhibiting low filterability (Lousada-Ferreira et al., 2010).
No relation between permeability and filterability has been reported in the studies developed by (Gil et al., 2011) using different membrane configurations (hollow fiber vs. tubular membranes) at full scale municipal WWTP in The Netherlands.
Fouling
Membrane fouling refers to the accumulation of materials on top of the filtrating surface affecting the permeate production. The accumulated materials can be both from inorganic or organic (mostly biological) origin (Henze, 2008). Even though MBR capital expenditures (CAPEX)s have been reduced as membrane costs have been continuosuy decreased, still the operational expenditures (OPEX) related to energy consumption (mainly to evercome fouling) represent the major constraint to implement large scale membrane bioreactors. Excesive fouling can lead to a relevant decrease in permeate production, an increase in the backflush/cleanning frequencies, and even to permanent damage due to irreversible fouling (Drews, 2010).
There are many ways to classify fouling depending on the fouling mechanism, the type of deposited material, and also based on the definitive or temporary character of the permeate losses.
Reversible fouling is associated to materials that can be removed from the membrane surface applying physical methods like relaxation or back flushing. Irreversible fouling can only be removed by chemical methods that are usually applied with a lower frequency (Henze, 2008); that condition allows irreversible fouling to build up as shown in Figure 2.6.
EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE
AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 23 Figure 2.6 Schematic of fouling rates in long term operation of full scale MBRs (Drews, 2010) Fouling is a very complex process depending on many factors as described extensively by (Le-Clech, Chen, & Fane, 2006) and (Judd, 2010). Fouling can be grouped in four main categories for submerged membranes as shown in Figure 2.7.
Figure 2.7 Factors affecting submerged membranes fouling. (Le-Clech et al., 2006)
EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE
AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 24 Figure 2.8 Inter relationships between MBR parameters and fouling. (Judd, 2010)
Cleaning and recovery
Currently applied cleaning techniques are basically divided in two groups, the physical and the chemical methods. They are both applied to remove different types of fouling materials. The cleaning frequency depends on the fouling nature and on the operational conditions such as the applied flux. The flux is usually fixed to obtain a constant permeate production. That is, the TMP will increase as the fouling builds up until reaching the maximum allowable TMP (Pmax, usually given by the membrane manufacturer). At this point
cleaning is required.
Physical methods are intended to remove reversible fouling. Backflushing and membrane relaxation are carried out in a timescale of minutes (1 to 2 minutes backflushing and/or relaxation every 10 to 30 minutes operation); they are highly dependent on the characteristics of the whole system (Wastewater-Sludge-Membrane-Operation regime).
Chemical cleaning of submerged membranes aims to remove irreversible fouling that cannot be removed by physical methods. Chemical cleaning uses different chemical solutions (organic acids and sodium hypochlorite) to take away the fouling layer and return the membrane to its original permeability state; however, the membrane never comes back to its original permeability values due to the presence of irrecoverable fouling.
EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE
AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 25 Figure 2.9 Simplified scheme of membrane cleaning methods. (Henze, 2008)
Chemical cleaning usually takes place two to four times per year, but it can be more frequent depending on the mixed liquor-membrane interactions. Cleaning in place (CIP) is the most common practice to perform chemical cleaning by filling the membrane chamber with the cleaning agent. CIP can also be performed on a separate tank.
2.3. CONVENTIONAL AERATION METHODS
Aeration is a key aspect in terms of operational expenses representing 45 to 75% of the total energy demand for aerobic treatment processes in both CAS and MBR treatment systems (Henze, 2008). Low pressure aeration with fine bubble diffusers is the most utilized popular method to deliver the system’s required oxygen demand. The bubble diffusers come in many different shapes and configurations; essentially, they are comprised of a structural part providing support to a perforated polymeric membrane that breaks down the air flow into small bubbles. Both disc and tubular diffusers are used extensively worldwide (Figure 2.10). Recently, flat panels are taking over a portion of the market due to their simplified requirements for installation and the fact that they can be easily removed from the aeration basin for either maintenance or replacement needs.
EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE
AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 26 The aeration efficiency depends on the temperature, the oxygen partial pressure at the site conditions, the amount and size of solids in the water, and on the effective contact area between the two phases (liquid and gas phase); therefore, the size of the bubble is relevant. The smaller the bubbles, the higher the contact area; therefore, the better the oxygen transfer.
2.3.1. Factors affecting oxygen transfer
ALPHA FACTOR
Sub surface aeration efficiency is often measured as the Oxygen transfer efficiency (OTE, %), As shown in Equation (2.7) the OTE is defined as the net oxygen mass flow into the water phase. (Henze, 2008).
, ,
,
(2.7)
The extent of the achievable transfer, among others, is affected by the amount and size of solids in the water (with a higher negative impact as the MLSS concentration increases). To evaluate the effects of the MLSS on the oxygen transfer, the alpha factor concept was developed. The alpha factor is defined as the ratio between the OTE on process water and the OTE on clean water as described in Equation 2.8 below (expressed for standard conditions of 20ºC and 1 atm) αSOTE and SOTE, respectively.
∝ ∝ (2.8)
Since the amount of oxygen transferred to the water phase is a function of a mass transfer process, the ratio between the process water mass transfer coefficient (KLa process water) and the clean water mass transfer
coefficient (KLa process water) is equivalent to the same alpha factor previously defined (Henze, 2008) as
described in Equation (2.9).
EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE
AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 27 The MLSS effect
The alpha factor is a representation on the extent of the reduction of the oxygen transfer in process water versus clean water. Therefore, it is easy to anticipate the big influence of the MLSS on this respect. That relation has been reported by many authors as shown in Figure 2.11.
Figure 2.11 MLSS concentration and alpha factor correlation (Ando, 2013).
This relation is especially relevant considering that the oxygen not only needs to get transferred through the water-liquid interphase, but also the dissolved oxygen needs to travel all the way until it reaches an active biofloc (Garcia-Ochoa & Gomez, 2009) as shown in Figure 2.12. The alpha factor is relevant for activated sludge processes, but even more relevant for membrane bioreactors where the high MLSS reduces the alpha factor introducing a big constraint to upscale the treatment capacities by increasing the applied organic load per unit volume of the biological reactor.
EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE
AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 28 Oxygen Uptake Rate (OUR20)
The oxygen not only needs to be dissolved in the water, but also the dissolved oxygen needs to be used by the active microorganisms to accomplish the purpose of aeration. The extent of the biological activity can be determined by the oxygen uptake rate (OUR); a parameter developed by many authors including (Sollfrank & Gujer, 1990), (Kappeler & Gujer, 1994) and (Spanjers, Vanrolleghem, Olsson, & Dold, 1998). The oxygen uptake rate (OUR20) is a measure of the oxygen depletion in a saturated sludge sample under
standard conditions (20ºC, 1 atm). It can also be referred as the respirometry test for the activated sludge. When divided the OUR by the sludge volatile suspended solids leads to the specific oxygen uptake rate (SOUR20); that is, a measure of the sludge activity per mass unit.
THE SPEECE CONE
The Speece cone was developed in the United States in 1971 for hypolimnetic aeration of ponds and lakes for remediation purposes by Richard Speece. Dr. Speece got a US patent, (Speece, 1972) but it was only until 1990 when it was actually evaluated at the Logan Martin Dam in Alabama by the Alabama Power Company (Ashley et al., 2008).
The Speece cone Figure 2.13 consists of a conic structure fed with water from the top at a high velocity (3 m/s). Pure oxygen is injected right at the inlet close to the smallest cone diameter. Once the oxygen is introduced, the gas buoyancy will lift the gas up, but the down-flow velocity of the water will keep the oxygen in the throat section of the cone. The oxygen bubbles have both enough detention time and surface contact area under mid-high pressure (from 2 to 5 bar) to get the oxygen dissolved into the water. Saturation oxygen concentrations of up to 300% can be achieved.
The supersaturated flow is discharged from the bottom of the cone to either a tank or a lake (or any other water source). There are no available research publications describing the performance of this technology on MLSS for membrane bioreactors aeration applications.
Figure 2.13 Speece cone. SOURCE: ECO2 TECH website 2014
The Speece cone has shown very satisfactory oxygen transfer efficiency results regarding both the oxygen transfer coefficient (KLa), and the standard oxygen transfer rate (SOTE) when the process is controlled
EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE
AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 29 Figure 2.14 KLa and SOTE increase with higher discharge velocities (Ashley et al., 2008).
EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE
AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 30 This chapter presents a brief introduction to the site where the pilot plant was installed. In addition, it describes the equipment that had to be assembled before the data acquisition phase started. Moreover, the analytical methods utilized on this research are described.
3.1. Worksite description
Since the Harnaschpolder wastewater treatment plant (HNP WWTP) is classified as an industrial facility, any person who works there is required to hold a special safety certification. The Veiligheids Checklist Aannemers (VCA), or safety checklist for contractors, can be obtained after an examination administered by a specialized company. The holder of this certification is considered to have enough knowledge to minimize the risks related to working in an industrial environment; that is, the worker can be responsible for his own safety. It was necessary to get the aforementioned certification before starting any kind of work at the treatment plant.
3.1.1. Harnaschpolder Wastewater treatment plant
The following information has been adapted from the DELFLUENT website's description of the HNP WWTP.
The Harnaschpolder Wastewater treatment plant is the newest plant in the area of South Holland. It was inaugurated in the year 2006, and it is the biggest plant in The Netherlands. In addition, the plant is amongst the largest WWTP in Europe with a treatment capacity of 1.3 million people equivalent (PE). The main relevant characteristics of this treatment plant are as follows:
Treatment capacity : 1.3 Million PE
: 3 m³/s Dry Weather Flow (DWF) 9 m³/s Rain Weather Flow (RWF) Occupied Area : 25.000 m²
Treatment units: : Coarse and fine screens (6mm)
Primary settlers (4 units; diam: 47m; h:3.5m)
Aerobic reactors (8 units)
Secondary clarifiers (16 units)
Sludge thickeners (2 units)
Sludge anaerobic digesters (2 units)
CHAPTER 3
EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE
AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 31 The plant uses a conventional activated sludge system (CAS) with coarse and fine screening followed by primary settlers. The primary influent is then pumped to the biological reactor consisting of a circular tank divided in different reaction chambers as follows: a selector, a pre denitrification chamber, and finally an aerobic section which also serves as a post denitrification basin when the aeration is turned off achieving anoxic conditions. The treatment train finishes with secondary settlers (Figure 3.2), and the treated water is pumped to a main storage facility in the proximity of the coastal area close to the treatment plant. From there, the treated wastewater is finally disposed three kilometers into the North Sea. The waste sludge is thickened and anaerobically digested to produce biogas. The biogas is used for power generation. The digested biosolids are dewatered with centrifuges and then transported by trucks to an incineration facility. Typical influent and effluent values are shown in Figure 3.1.
Figure 3.1 Influent and effluent water quality at Harnaschpolder WWTP
Figure 3.2 General view of the secondary settlers HNP WWTP.
January February March April May June July August SeptembeOctober Novembe Decembe Q m³/d 224,226 217,828 180,786 169,456 189,603 160,677 175,062 215,437 151,883 180,177 177,835 228,907 T °C 13.5 13.1 14.3 16.2 17.0 22.4 20.2 20.4 19.4 17.9 14.6 pH 7.7 9.8 9.7 8.8 8.3 8.2 4.6 7.1 6.3 5.7 5.6 7.2 TSS mg/l 228.7 246.8 267.0 276.7 272.9 306.3 285.0 239.7 289.3 286.3 291.9 252.3 BOD5 mg/l 208.0 211.6 255.7 265.0 251.6 271.0 240.0 189.0 264.4 234.7 280.4 213.9 COD mg/l 448.0 487.2 556.3 578.8 544.8 587.5 559.3 441.0 579.1 558.3 583.3 480.8 0.46 0.43 0.46 0.46 0.46 0.46 0.43 0.43 0.46 0.42 0.48 0.44 TKN mg/l 46.3 50.2 57.4 60.3 54.6 58.3 55.1 43.0 59.8 55.7 58.0 47.2 Ntot mg/l 46.3 50.2 57.4 60.3 54.6 58.3 55.1 43.0 59.8 55.7 58.0 47.2 PT mg/l 6.5 7.3 8.1 8.6 7.9 8.6 8.1 6.3 8.8 8.1 8.3 6.8 PE 1,041,223 1,053,669 1,067,336 1,058,533 1,076,416 1,006,888 1,003,727 922,304 934,001 1,069,619 1,077,613 1,074,301 2014 influent
January February March April May June July August SeptembeOctober Novembe Decembe
EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE
AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 32 3.1.2. Pilot Hall
At the facilities of the Harnaschpolder WWTP, a hangar was recently built fully dedicated to conduct research activities. The pilot hall (Figure 3.3) has approximately 200 square meters. Several research and educational institutions such as the Technical University of Delft (TU Delft) and, for the first time, UNESCO-IHE are allowed to evaluate pilot scale equipment at real conditions. One of the big advantages of performing research at this facility is the unlimited availability of raw influent and also activated sludge which was crucial for this research.
Figure 3.3 Harnaschpolder WWTP pilot hall view
3.2. Equipment description
3.2.1. Pilot MBRA package MBR unit with an installed hydraulic capacity of 1 m³/d was used as the pilot MBR reactor during this research. The reactor is provided with a stainless steel tank divided in four main compartments as follows:
Anoxic Tank: The screened influent (through a 0.45µm screen) is pumped in to this tank; from there, the influent overflows to the aerobic tank. This tank has an effective volume of 0.3 m³.
Aerobic Tank: This is where most of the organic load removal occurs; the mixed liquor reacts in presence of oxygen to produce a treated effluent that can be extracted though the membrane module. This tank is provided with the following ancillary equipment:
Fine bubble diffuser: to supply air in a conventional way. The air flow is provided using a low pressure blower.
EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE
AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 33 depending on the operation mode (according to the manufacturer information – Memos). This membrane module is shown in Figure 3.4.
Figure 3.4 MBR Membrane module
Recirculation pump:It is used to recirculate the mixed liquor from the aerobic tank to the anoxic tank, for the denitrification process to occur. A fraction of the recirculated flow can be discharged as waste sludge by using a set of two ball valves. The pump runs automatically with a timed control loop controlled by the main programmable logic controller (PLC).
Permeate tank: The permeate tank receives the treated water (permeate), which is also used for the membrane backwash. The permeate tank has a free discharge inlet (θ 25mm), a drain line with a ball valve at the bottom (θ 25mm), and an outlet line (θ 25mm).
Waste sludge tank: The waste sludge tank is a separate compartment built in the MBR for temporary storage of wasted sludge. The tank has a discharge line (2 inches) to the drain and a ball valve to control the discharge.
Permeate Pump: A bidirectional centrifuge pump is used for both the permeate production and the backwash processes. The pump can deliver a maximum flow of 1 m³/h with a dynamic head of 10 m. The operation of the pump is conditioned by the level sensors signals controlled by the PLC to perform time controlled cycles
EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE
AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 34 Figure 3.5 Low pressure blower for conventional aeration
Control and instrumentation
Control box: NEMA4 box with a main PLC to control the operation in automatic mode as shown in Figure 3.6.
Figure 3.6 MBR control box.
EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE
AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 35 Figure 3.7 Electromagnetic flow indicator transmitter (FIT)
Pressure indicator transmitter (PIT): The PIT is installed in the permeate line. This instrument gives a 4-20 mA signal to the PLC. In addition, this instrument protects the membrane from over pressure. The setpoints are established at +50 and -55 Kpa when backwashing and permeating, respectively Level switches: Double plate contact switches built in the aerobic tank wall. There are three in total
for high level (H), low level (L) and very low level (LL) to control the feed pump, permeate pump, and permeate cycles.
3.2.2. Speece cone setup
A Speece cone setup is coupled to the MBR system. The Speece cone system consists of a positive displacement pump and a pressurized cone. The mixed liquor is pumped from the aerobic tank through a pressurized cone. At the pressurized cone, oxygen gas is introduced and gets dissolved in the mixed liquor. Then the mixed liquor is returned back to the aerobic tank. The process components are shown in Figure 3.11. The main components of the Speece cone system are described as follows:
Sludge pump: The sludge pump (Figure 3.9) has a maximum pumping capacity of 12 m³/h at 7 bar. The pump can be controlled by using a variable frequency drive (VFD, Figure 3.8). A control loop with a flow meter was incorporated to stop the pump when the flow is too low. The control loop avoids potential damage to the stator by preventing the system to run dry.
EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE
AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 36 Figure 3.9 Positive displacement pump for the sludge closed circuit. It had to be assembled on site.
Speece cone: The cone is a pressure vessel where the oxygen dissolves into the mixed liquor in a closed recirculation circuit as observed in Figure 3.10.
EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE
AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 37 A simplified process flow diagram for the MBR, Speece cone, and external carbon source addition is shown in Figure 3.11
Figure 3.11 Equipment layout schematic
3.2.3. The Delft Characterization Method Installation (DFCi)
The DFCi (Figure 3.12 and Figure 3.13) is a complex analytical set up consisting of a single UF cross flow membrane element where the activated sludge is continuously filtered in a closed circuit. The feed, permeate, and concentrate pressures are continuously measured to determine the TMP during the performance of test. The DFCi reports the filterability (ΔR20[x1012m-1]) of the sludge (which is a property
of the sludge). The filterability represents the added resistance to filtration of the sludge, as a sludge cake layer builds up on top of the membrane. The added resistance value (ΔR20) is calculated after producing 20
L/m² of permeate through the membrane at a specific flux of 80 LMH and at a crossflow velocity of 1 m/s. The DFCi consists of the following ancillary equipment:
EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE
AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 38 Figure 3.12 The Delft Filtration Characterization installation at the TU Delft.
EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE
AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 39 3.2.4. Other sub-systems
Oxygen gas feed system
The setup is described in Figure 3.14 and Figure 3.15. It consists of an oxygen gas cylinder (50 L at 200 bar) followed by a pressure regulator to bring down the cylinder's pressure from 200 to 5 bar. Then, a needle valve allows regulating more precisely the pressure to values lower than 5 bar. Following the needle valve, a bypass valve for pressure release was installed. This valve also serves as an oxygen source to saturate a sample when performing the OUR determination. The system is also provided with a digital mass flow controller; this controller allows to have a steady gas feed to the injection point at the Speece cone. A gas relief valve is provided downstream the mass controller in the proximity of the gas inlet at the cone for easy depressurizing the mass flow controller. The relief valve is followed by both a ball valve and a check valve to avoid the sludge from entering the gas line.
EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE
AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 40 Figure 3.15 Oxygen gas feed system components
Alternative Carbon source feed system
An additional setup was assembled in case an external source of organic load is needed to sustain the growth of high concentration biomass. The setup consists of a 200 L tank, a standing mixer, and a peristaltic pump as shown in Figure 3.16 below.
EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE
AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 41 Biological Oxygen Monitor (BOM) system
A peristaltic pump takes the mixed liquor directly from the reactor and to the BOM. The BOM is sitting on a magnetic plate keeping the sample homogeneous. A dissolved oxygen probe can be introduced into the BOM to continuously measured and log the dissolved oxygen values to later calculate the oxygen uptake rate.
Activated Sludge feed system
Considering that large amounts of activated sludge from the WWTP will be required for seeding and starting up the 1.2 m3 MBR reactor, a feed system (Figure 3.17) was installed to bring sludge from the
WWTP to the pilot MBR system. A submersible pump was installed in one of the sludge pumping stations at the HPN WWTP. This pump was placed in the activated sludge chamber of the HPN WWTP tank E30 (approximately 150 m far from the pilot hall); from there, the sludge is pumped through a HDPE piping system from the plant to the pilot hall. This installation is temporal, and the pump has to be switched on and off manually. This setup will be replaced by HPN WWTP staff for a permanent pump to make the activated sludge available at the pilot hall for research purposes.
Figure 3.17 Submersible pump in the sludge basin connected to the sludge line (Up left); Sludge line at the pilot hall connected to a reinforced hose (Up right); Reinforced hose and MBR reactor (down left); Activated sludge discharge in the
EVALUATION OF A PILOT MBR SYSTEM OPERATED AT HIGH MLSS PROVIDED WITH A SPEECE CONE
AERATION SYSTEM AS AN ALTERNATIVE FOR SANITATION PROVISION IN EMERGENCIES 42
3.3. Analytical methods
The entire water quality analyses were carried out following the standard methods procedures for water and wastewater analysis using the materials described in Table 3.1 (Awwa, 1998).
The OUR was measured according to the EPA method 1683 (Specific Oxygen Uptake Rate in biosolids) Filterability
The sludge filterability was evaluated using the Delft Filtration Characterization method (DFCm) developed at the Delft University of Technology (TUD). With this method it is possible to calculate the added resistance the sludge opposes to filtration after producing 20 liters of permeate per square meter of membrane. A sludge sample (approximately 30 L) is filtered through a single membrane element at a crossflow velocity of 1 m/s and at a permeate flux of 80 LMH (Lousada-Ferreira, 2011).
The filtration resistance can be calculated according to the following equation (Eq. 3.1):
/ ²
(3.1)
The controlled operational conditions at which the filterability test takes place allow to calculate the flux (J), the trans membrane pressure (TMP) and the dynamic viscosity (ηp), knowing that it is possible to
calculate the total resistance (Rt) value using Equation 3.1. Then, the resistance values obtained along the
test are plotted against the specific permeate production (Vs) (see Figure 3.18) and using a power law regression it is possible to obtain a mathematical expression as follows:
∆ (3.2)
Figure 3.18 DFCi output file example with the added resistance against the specific permeate production.
Where "a" and "b" are the known absolute coefficients describing each filtration test obtained from the previously described plot and regression. Then using a fixed permeate production value (Vs=20 L/m²), the
ΔR20 values can be calculated as follows:
∆ 20 (3.3)
The cake layer filtration theory (Equation 3.4) describes the added resistance (ΔR) with an expression that matches the form of Equation 3.2 as follows:
y = 0.0921x1.1373 R² = 0.9905 0.01 0.10 1.00 10.00 1.00 10.00 100.00 Ad d. R esi st an ce [ *1 0 12 m -1] volume [l/m2]
