HERIOT-WATT UNIVERSITY
School of the Built Environment
D21WW (Water & Wastewater Treatment)
Recommended Reading Materials
Texts
Gerard, K (1996). Environmental Engineering. McGraw-Hill.
Haigh N (1987): EC Environmental Policy & Britain. 2/e, Longman (A loose leaf continuously updated edition is held in the library
Horan, N J (1990). Biological wastewater treatment systems (theory and operation). J. Wiley.
Metcalf and Eddy Inc. (1991). Wastewater Engineering: treatment, disposal, reuse. McGraw-Hill.
Tchobanoglous, G and Schroeder, E D (1987). Water quality, Addison-Wesley. Gray, N F (1994). Drinking water quality. J. Wiley.
Journals
Syllabus
Year 4 Semester 2 School of the Built Environment
Module Title(s) Water and Wastewater Treatment
Module Code(s) D20WW
Outline syllabus of module(s)
PART A: Introduction
Need for water & wastewater treatment Aim of the module
Overview of water and Wastewater quality characteristics
PART B: Engineered Systems for Water Treatment Introduction to different treatment combinations Aeration
Solids separation- the theory Primary sedimentation & design Coagulation & flocculation Filtration
Disinfection and disinfecting agents
Removal of refractory organics, colour and odour
PART C: Engineered Systems for Wastewater Treatment Overview of available options
Preliminary Treatment Design
Primary Treatment: a re-statement of primary sedimentation Secondary Treatment
Further Wastewater Treatment
Sludge Handling, Treatment and Disposal Disposal of Wastewater Treatment Effluent
PART D: Legislative aspects of Water Pollution Control UK laws
EU Directives Future direction
D20WW: Water and Wastewater Treatment
Part A: Introduction and Water Quality
Characteristics
1. Introduction
Raw water from the different sources- rivers, springs, aquifers, etc- contains several impurities that must be removed before the water becomes suitable for drinking. However, because of the variation in the nature and levels of pollution, the combination of treatment process applied differs.
On the other hand, the wastewater resulting from applying water to various domestic, industrial, commercial and agricultural uses contains many impurities that must be removed before the wastewater is discharged into the environment. The essence of wastewater treatment therefore is the substantial removal of major pollutants from the wastewater before discharging it into the environment. It is impossible to remove all of the pollutants; however, if a substantial part, typically >80%, is removed before discharge, then little or no adverse effects will be caused in the environment.
2. Aim
The aim of this module is to understand the basis of design of water and wastewater treatment plant processes and operations, including the safe disposal of final products, i.e. effluent and sludge, of the treatment. The module will conclude with a brief discussion of the legislative and other provisions for water pollution control as applied in the UK and elsewhere.
3. Important Water and Wastewater Characteristics – an Overview
The pollution loading of wastewaters from individual trades, farms etc., may be expressed in terms of kg/day of particular compounds. However, for mainly domestic wastes, the three most common parameters used to describe the waste are biochemical oxygen demand (BOD), the suspended solids (SS) and the bacteriological quality.
3.1 Biochemical oxygen demand (BOD)
(a) Carbonaceous BOD (CBOD)
This is the oxygen used up by the bacterial oxidation of organic pollutants (Carbonaceous BOD) or ammonia (nitrogenous BOD), usually determined under standard conditions of incubation of 20 oC over 5 days. (Generally in this case, the nitrogenous BOD will be small, simply because those bacteria which oxidise ammonia would not have developed in sufficient numbers by the fifth day. The nitrogenous bacteria will become very abundant after about 7-8 days when most of the carbonaceous organic pollutants have been removed.) A BOD of 10mg/l means that 1 litre of waste will require 10 mg of oxygen when incubated at 20 oC for 5 days. The sample is incubated in a sealed bottle (250 - 300 mls) ; the initial dissolved oxygen concentration (measured on a duplicate sample ) less the final dissolved oxygen concentration (measured on the incubated sample) gives a measure of the BOD. The necessary bacteria and nutrients may be present naturally or may be added.
Often it is necessary to predict the BOD without actual measurement. This can be achieved by modelling the BOD decay process. The kinetics of BOD oxidation depend upon the particular organics present; however, for domestic waste a first-order reaction rate is assumed:
dL
dt
= −
K L
1
(1)
where L is the BOD remaining or unoxidized (mg/l), t is time (d) and K1 is a constant
with units of T -1 (day -1). Integration of the above equation using Lo as the initial
unoxidized BOD gives the unoxidized BOD after time t as
Lt
Lo
= −
e K t
1
(2)The oxidized BOD then becomes
BODt
=
Lo Lt
−
=
Lo
1
− −
e K t
1
(3)Lo is often referred to as the ultimate BOD (mg/l), and approximates to the result of a
BOD test carried out at 20oC over >20 days. Also, K1 depends on temperature in the
form:
KT
=
K
20
θ
T
−
20
(4)where KT and K20 represent the values of K1 at T oC and 20 oC, respectively and θ is a
constant.
(b) Nitrification and Nitrogenous BOD (NBOD)
The term BOD in general calculations on loading, sewage treatment, effluent standards etc. usually means the 5 day 20oC BOD exerted by the carbonaceous material. In most circumstances this is acceptable because, as stated previously, the nitrosomonas and nitrobacter species of bacteria which are responsible for oxidizing (ammonia)
NH
4
→
NO
2
−
(nitrite) andNO
2
− →
NO
3
−
(nitrate), respectively, byNITRIFICATION are usually in very low concentrations in most samples of rivers,
percolating filter, the nitrogenous oxygen demand may be partially exerted within the 5 day period. Nitrogenous kinetics are also assumed to be first-order and the resulting BOD expression is
BODN
=
LNO
1
− −
e KN t
(5)where LNO is the ultimate nitrogenous BOD (mg O2/l), KN is the reaction rate
constant (day-1), and t is time in days. KN, like K1, also depends on the temperature
(see eq. (4)).
For a combined carbonaceous BOD plus nitrogenous BOD, the total BOD is
(
)
BOD
=
Lo
1
− −
e K t
1
+
LNO
1
− −
e
KN t t N
−
(6)where tN is a delay time (day) before nitrifying organisms are effective.
Example 1
A BOD test is carried out for 15 days at 20oC. The reaction rate constants K1 and KN
are 0.16 day -1 and 0.1 day -1, respectively, and Lo and LNO are 450 mg/l and 300
mg/l, respectively. The nitrifying organisms are assumed to be effective only from the time t = 8 days. Calculate the BOD exerted at days 5, 8, 12 and 15.
Solution
Limitations of the BOD
The problems with the BOD as a water quality parameter include that: i. its determination takes a long time- at least 5 days for current standard;
ii. its determination may be affected by the presence of bactericidal substances in the sample;
iii.it merely characterises the biodegradable organics; wastewater, particularly from industrial sources, contain a whole range of non-biodegradable organics which also need oxidising with dissolved oxygen.
Hence, there is now the move away from BOD to using the chemical oxygen demand (COD). The determination of the COD is much faster (3 hours) and characterises all oxidisable substances in the waste, whether or not they are biodegradable.
Example 2
A BOD test was carried out on three samples of river water. All gave a dissolved oxygen initial reading of 7 mg/l. The final dissolved oxygen reading after 5 days incubation at 20 oC were 0, 3, 7 mg/l. Calculate the three BOD values.
Solution
3.2 Suspended solids (SS)
The material in suspension in sewage commonly consists of inorganic grits, silts or clays, organic compounds such as bacteria, fats, greases, and a wide range of wastes from food cleaning and preparation processes. The total suspended material which will normally be removed by passage through a fine glass-fibre filter paper is classed as suspended solids.
Both the BOD and SS concentrations in sewages are influenced by the daily per capita water usage. Present practice is therefore to assume a per capita production of BOD and SS in terms of kg/cap.day and to calculate expected concentrations in terms of local water usage.
Example 3
A small sewage works serves a population of 500 people whose per capita BOD5 and
SS production is 0.055 kg/day and 0.08kg/day, respectively. Water consumption is metered at 200 l/cap.day. A small industry discharges an effluent of 5 l/s containing 100 mg/l BOD5 and 200 mg/l SS over a period of 3 hours per day. Calculate the mean
Solution
In general, both the SS and BOD5 are the two main parameters for monitoring the
performance of treatment works. In the UK for example, it is customary to expect that effluents from treatment works contain no more than 20 mg/l BOD5 and 30 mg/l SS
for discharge to inland rivers, based on the Royal Commission Standards. These are referred to as the consent conditions.
3.3. Turbidity
While the SS is used for wastewater, turbidity is used for examination of water. Turbidity is the extent to which light penetrates in the water sample. The main source of turbidity in water is the erosion of colloidal materials e.g. clay, silt. Other sources are soaps, detergents in household and in industrial wastewater.
Turbidity is measured photometrically using the Jackson Turbidimeter (Figure 1) and expressed JTU (Jackson Turbidity Unit). 1 JTU (Jackson Turbidity Unit) = Turbidity of 1 mg SiO2/litre of distilled water
More recent unit of measurement is the FTU (Formazin Turbidity Unit). Formazin provides more reproducible results/standards than SiO2; the standardised candle has also now been replaced by electric bulb.
View Sample
Long glass tube (calibrated against turbidity by silica, SiO2)
Black metal sheath
Standardised candle
3.4 Bacteriological standards
Some countries use bacteria as an indication of the pollution state of a water sample. However, because of the very many types of bacteria and other micro-organisms that could be present, the practice is to use certain bacteria derived from the gut of warm blooded animals as an indication of pollution, e.g. faecal coliforms. The number of these micro-organisms is enumerated using the fermentation tube test. Five tubes containing 10ml, five tubes containing 1 ml, and five tubes containing 0.1 ml of sample together with the necessary growth medium are incubated for 24 hours and the number of tubes showing positive reactions, indicated by the giving off of gas, is determined. Then the number of bacteria is expressed as most probable number, MPN/100 ml sample which is read from tables prepared for the purpose. Note that for a strong sewage, extensive dilution of the sample may be necessary prior to the incubation.
3.5 Typical Sewage Analyses
An analysis of a typical municipal sewage will produce characteristics as shown below. The wastewater will also contain other quality parameters as shown in Table 2.
Treatment Stage Quality characteristics (mg/l) Crude sewage (Arriving at works) Settled sewage (After primary clarifier) Final effluent BOD 300 175 20 COD 700 400 90 SS 400 200 30 Ammonium-N (NH4-N) 40 40 5 Nitrate-N (NO3-N) <1 <1 20
3.6 Overall Composition of Raw Sewage
Thus, sewage is 99.9% water but only 0.1% solids. The implication of this is that large volumes of water must be moved around during wastewater treatment, in order to remove only a small quantity of solid impurities. As will be seen later on,
significant savings in the size of wastewater treatment facilities can be had by reducing the amount of water contained in sewage. Indeed, this is usually one of the main objectives in the treatment and handling of sewage sludge.
3.7 Simple Rules for Handling Water Quality characteristics
1. Concentrations of water quality variables are normally expressed as milligrams/litre (or simply mg/ l ). In general, 1 mg/ l is equivalent to 1 gm/m3 or 10-3 kg/m3. Also, 1 mg/ l is the same as 1 ppm (or parts per million). When concentrations are very high, they are often expressed in %, where 1% is equivalent to 10,000 mg/ l .
2. The load due to a given water quality characteristic is obtained by multiplying its concentration by the flow rate, i.e.
Raw Sewage 99.9% 0.1% Water Solids 70% Organic 30% Inorganic grit salts metals Protein fats Carbohyd-rates
Q C
L= × (7)
where L is the load, C is the concentration and Q is the flow rate. If the concentration is expressed in kg/m3 and the flow in m3/day, then the load will be in kg/day.
3. The above equation (1) is very useful for combining water quality characteristics from several sources/activities. For example, the inflow to a wastewater treatment works can come from several sources: domestic houses, commercial activities, industrial activities, agricultural activities, etc. Each of these activities/sources has different levels of water quality characteristics and it will be necessary to obtain the composite concentration, after they are mixed together, to use in designing the works. Obtaining the composite concentration for each water quality characteristic is done as follows:
• Compute the load for the characteristic of interest (e.g. BOD, SS, etc) in each of the activities/sources using equation (1)
• Compute the total load by adding all the individual loads.
(IMPORTANT: WHILE YOU CAN ADD LOADS, YOU MUST NEVER ADD CONCENTRATIONS!!)
• Compute the total flow by adding the flows from each of the activities
• Divide the total load by the total flow to obtain the composite concentration. In other words,
∑
∑
= = = N 1 i i N 1 i i composite Q L C (8)where Ccomposite is the composite concentration, Li is the load due to source i,
Qi is the flow due to source i and N is the total number of sources.
For equation (2) to be valid, all flows must be in the same unit and all concentrations must be in the same unit. In other words, it makes no sense if the flow from the houses is expressed in m3/day while that from the industry is expressed in l /sec OR if the BOD from the houses is in mg/ l but the BOD from the industry is in kg/m3. You must make all the necessary conversion and ensure that all the water characteristics (flow and quality) are expressed in similar units across all the relevant sources/activities.
Table 1: Water Quality Parameter Groups and their significance in Wastewater Treatment (after Metcalf and Eddy, 1991)
CONTAMINANTS REASON FOR IMPORTANCE
Suspended solids Suspended solids can lead to the development of sludge deposits and anaerobic conditions when untreated wastewater is discharged in the aquatic environment.
Biodegradable organics Composed principally of proteins, carbohydrates and fats, biodegradable organics are measured most commonly in terms of BOD (biochemical oxygen demand) and COD (chemical oxygen demand). If discharged untreated to the environment, their biological stabilization can lead to the depletion of natural oxygen resources and to the development of septic conditions. Pathogens Communicable diseases can be transmitted by the
pathogenic organisms in wastewater, such as bacteria, viruses, helminths, etc..
Nutrients Both nitrogen and phosphorus, along with carbon, are essential nutrients for growth. When
discharged to the aquatic environment, especially the often limiting phosphorus and nitrogen compounds, these nutrients can lead to the growth of undesirable aquatic life, resulting in the rapid ageing of stagnant or slow moving water bodies in lakes and rivers. This rapid ageing phenomenon is known as eutrophication. When discharged in excessive amounts on land, nutrients can also lead to the pollution of groundwater.
Priority pollutants Organic and inorganic compounds selected on the basis of their known or suspected carcinogenicity, mutagenicity, teratogenicity, or high acute toxicity. Many of these compounds are found in wastewater.
Refractory organics These organics tend to resist conventional methods of wastewater treatment. Typical examples include surfactants, phenols, and agricultural pesticides.
Heavy metals Heavy metals, e.g. lead, zinc, are often added to wastewater from commercial and industrial activities and may have to be removed if the wastewater is to be reused.
Dissolved inorganics Inorganic constituents such as calcium, sodium, and sulphate are added to the original domestic water supply as a result of water use and may have to be removed if the wastewater is to be reused.
Table 2: Typical Water Quality Parameters for Fresh, Untreated Domestic Wastewater (after Metcalf & Eddy, 1991)
o Values should be increased by amount present in domestic water supply.
# Usual COD/BOD ratio is approximately 2.0
~ This is for fresh wastewater. As time progresses and the wastewater comes into contact with air, the ammonia form of the nitrogen is gradually oxidised into the inorganic forma (nitrite-N and nitrate-N) by a process known as NITRIFICATION.
Concentration
Contaminants
Unit
Weak
Medium
Strong
Solids, total (TS) Dissolved, total (TDS) Fixed Volatile Suspended solids (SS) Fixed Volatile mg/L 350 250 145 105 100 20 80 720 500 300 200 220 55 165 1200 850 525 325 350 75 275 Settleable solids (e.g.
grit) mL/L 5 10 20 Biochemical oxygen demand, mg/L: 5-day, 20oC (BOD5. 20 o C) mg/L 110 220 400
Total organic carbon (TOC) mg/L 80 160 290 Chemical oxygen demand (COD) # mg/L 250 500 1000 Nitrogen (total as N) ~ Organic Free ammonia Nitrites Nitrates mg/L 20 8 12 0 0 40 15 25 0 0 85 35 50 0 0 Phosphorus (total as P) Organic Inorganic mg/L 4 1 3 8 3 5 15 5 10 Chlorideso mg/L 30 50 100 Sulphateo mg/L 20 30 50 Alkalinity (as CaCO3) mg/L 50 100 200
Grease mg/L 50 100 150
Total coliformo No./100 ml 106 - 107 107 – 108 107 – 109 Volatile organic
compounds (VOCs)