As stated earlier, lagoons systems work most effectively in a two basin operation (aeration and settling). Where land area is not available for two basins, or a existing single pond system is to be upgraded, the single lagoon can be partitioned with a barrier. The first zone would be aerated allowing for substrate removal while the second zone would allow for solids settling.
The need for a liner for both the aeration and settling zones should be assessed when upgrading or installing a new lagoon system. This decision is a site specific one depending on local environmental guidelines, soil permeability, and location and use of underlying groundwater.
8.0 AERATED LAGOONS (Cont) Aeration Zone
Feed characterization is an important step in the design of any biological oxidation system. Although lagoon systems are capable of treating a wide range of wastewaters, their efficiency is affected by a number of factors such as free oil, sulfides, cyanides, etc. For maximum efficiency, the influent should be of the same quality required for an activated sludge plant.
Limiting concentrations of a number of constituents for activated sludge plants are given in Table 6.2-1. The lagoon may operate at higher loadings than those displayed, but at a reduced efficiency. Additional information on feed requirements for biological treatment systems is given in the Activated Sludge subsection of this Design Practice.
Due to lack of sludge recycle (lagoon HRT = SRT), the aeration zone of a lagoon system is less efficient than activated sludge processes, achieving 60 to 90 percent BOD5 reduction and 80 to 95 percent phenol reduction. The temperature in the aeration basin can also affect substrate removal efficiency. Recommended temperatures for biological treatment are between 50° and 105°F (10 to 40°C). In general, the rate of substrate removal is doubled for every 18°F (10°C) temperature rise in this range.
More detailed information on the affect of temperature on biological systems is given in the Activated Sludge subsection of this Design Practice (Section 6.0).
Aerators serve two purposes in an aerated lagoon system: to meet oxygen requirements for substrate removal and to provide the necessary power requirements for complete mixing and solids suspension. Suspended biosolids concentrations in the aeration zone of a lagoon system are lower than in an activated sludge system (2500 to 6000 TSS for activated sludge vs.
100 - 350 mg/L TSS for an aerated lagoon). Although it is not necessary to maintain total solids suspension in an aerated lagoon, it is important to provide sufficient power for complete mixing. In dealing with dilute wastes, or in lagoons with long retention times, it is possible for the horsepower requirements for mixing to exceed the horsepower required for oxygen transfer. Estimating power requirements for both oxygen transfer and complete mixing will be discussed below in the design procedure section.
Figure 4.2-1 of DP Section XIX-A6 presents a decision tree for aeration selection. Due to the relatively shallow basin depth usually encountered in a lagoon system (6 to 12 ft), mechanical surface aerators are often used. A complete discussion on aeration systems, including depth requirements and circles of influence for complete mixing, is given in the Aeration subsection of this Design Practice.
For screening purposes, a residence time of one day in the aeration section will usually allow for adequate substrate removal.
A depth in this section of 6.6 ft (2 m) can also be used as a first pass estimate in sizing the basin. Other rules of thumb for aeration, mixing, and settling requirements, will be discussed throughout this subsection. These numbers should be used as a fist pass for screening estimates only and not for use in detailed design.
Settling Zone
If the suspended solids concentration from the aeration zone exceeds permitted discharge levels, a settling zone must be provided. Usually, settling is accomplished in a large, relatively shallow earthen basin. In designing a settling basin, the following issues should be addressed: (1) adequate retention time must be provided to achieve the desired degree of suspended solids removal (2) sufficient volume must be provided in the basin for sludge storage (3) algal growth must be minimized (4) odors that may develop as a result of the anaerobic decomposition of the accumulated sludge must be controlled.
A minimum retention time of one day is usually required to settle the majority of the settleable suspended solids. Adequate volume must be factored into the design to allow for sludge storage so that accumulated solids will not significantly affect retention times. Periodically, the settled sludge may need to be removed in order to maintain minimum retention times and control odors in the basin.
Extended retention times in the settling basin can promote algal growth, which can contribute to effluent TSS and BOD levels.
Although the growth rate is site specific, depending on available nutrients and sunlight, algae can usually be controlled by limiting the retention time in the settling basin to within 2 - 10 days. Odors arising from anaerobic decomposition can be usually be minimized by maintaining a minimum water depth of 1 meter in the settling basin. In warmer climates, a deeper basin may be needed.
For screening purposes, a residence time of five days in the aeration section with a depth of 6.6 ft (2 m) will usually allow for adequate solids settling as well as minimal odors and algal growth. These numbers should be used as a first pass for screening estimates only and not for use in detailed design.
8.0 AERATED LAGOONS (Cont) 8.4 DESIGN PROCEDURE
Substrate Removal
First order kinetics can be assumed for substrate removal in an aerated lagoon. The following equation is used to determine an appropriate retention time in the aeration section of the lagoon system. The organic substrate, S, is usually expressed in terms of BOD5 but COD can also be used when BOD5 is not available.
S / So = 1 / (1 + q HRT) Eq. (8.4-1)
where: S = Effluent substrate concentration (mg/L) So = Influent substrate concentration (mg/L) q = Rate of substrate removal (1/d)
HRT = Hydraulic residence time in the aeration basin (d)
A typical value for q is given in the sample problem below (Section 8.5). In cases where systems are being designed to accommodate facility expansions or as part of a grass roots plant, actual wastewater flowrate and concentration data may not be available. If this is the case, load factors can be estimated using ER&E Report No. EE.86E.86 Refinery Process Unit Wastewater Load Factors - Final Report.
Once the required HRT is defined, flowrate can be used to calculate basin volume. Basin depth can then be used to determine the surface area needed for the aeration zone.
Temperature Effects
As discussed earlier, system temperatures can have a significant effect on substrate removal and must be considered in the design of an aerated lagoon system. Temperature effects are discussed in more detail in the Activated Sludge subsection.
The relationship relating temperature and removal rate is stated as:
qtd = (q20)[(1.08)(td-20)] from Eq. (6.3-2)
where: qtd = Rate of substrate removal at design (minimum) temperature, td °C (1/d) q20 = Rate of substrate removal at 20°C (1/d)
td = Design temperature, °C
Oxygen Requirements
In order to provide ample oxygen for treatment, a basis of 1.2 times the total influent chemical oxygen demand (COD) is often used as a rough rule of thumb. A more detailed explanation on determining oxygen requirements is given in the Aeration subsection of this DP.
Power Requirements for Oxygen Transfer and Solids Suspension
After the oxygen requirement for the aeration basin has been determined, the total basin aerator horsepower (for surface aerators) can be calculated using an oxygen transfer rate of 2 lb O2/ bhp-hr (0.9 Kg O2/bhp-hr). The HP needed for solids suspension is then calculated. As stated earlier in this subsection, less power is required to keep solids in suspension in an aerated lagoon than in an activated sludge system. As a rule of thumb, 60 to 100 HP/million gallons (16 to 26 HP/1000 m3) is sufficient to provide power for complete mixing and solids suspension.
The number of aerators should be chosen so that the total HP is spaced evenly throughout the aeration zone, maintaining complete coverage without creating any dead zones. If surface aerators are used, they should be placed such that the complete mixing zones are just about touching but the impingement pattern diameters are not overlapping. Surface aerators are also associated with a minimum depth requirement (increasing depth with increasing HP). Refer to DP Section XIX-A6 (Aeration Systems for Biological Treatment) for more details on surface and diffused aeration.
Sludge Accumulation
If sufficient power is supplied to the aeration section of the lagoon system, there should be minimal solids accumulation in this area. There will, however, be solids accumulation in the settling basin. For screening purposes, an estimate of 530,000 lb (240,000 Kg) of sludge will be produced per year per million gallons of wastewater flow can be used. Sludge accumulation is highly variable and depends on several site specific factors such as substrate removal, influent TSS, and effluent quality. It is recommended that the following calculations be used to more accurately determine the rate of sludge accumulation:
8.0 AERATED LAGOONS (Cont)
SS = SSo + Xv / 0.8 Eq. (8.4-2)
where: SS = Suspended solids in the aeration basin effluent (mg/L) SSo = Suspended solids in the aeration basin influent (mg/L)
Xv = Lagoon volatile suspended solids (mg/L) [Xv/0.8 = suspended solids contribution excluding influent suspended solids]
Xv can be calculated using Eq. (6.1-12). For the aerated section of the lagoon, the hydraulic residence time is equal to the biomass sludge retention time (HRT = SRT) making this equation:
Xv = ((So - S) Ym ) / (1 + b HRT) Eq. (8.4-3)
where: S = Effluent substrate concentration (mg/L) So = Influent substrate concentration (mg/L)
HRT = Hydraulic residence time in the aeration basin (d) Ym = Maximum microbial yield coefficient, dimensionless b = Endogenous decay coefficient, d-1
Determining values for Ym and b with and without pilot data are discussed in detail in the Activated Sludge Section of this DP (Section 6.0).
The amount of sludge that will settle in the basin over one years time can then be calculated as:
Sludge = (SS - SSe) Q (8.34 lb/Mgal ⋅ (mg/L)) 365 day/yr Eq. (8.4-4) where: Sludge = Sludge accumulation in settling basin per year (lb)
SS = Suspended solids in the aeration basin effluent (mg/L) SSe = Suspended solids in the settling basin effluent (mg/L) Q = Design wastewater flowrate (Mgal/d)
This is a conservative estimate of accumulation in that it does not take degradation of the settled sludge into account. In reality, some of the settled biomass (30% - 60%) will degrade over time. In the absence of data, a 40% reduction of the biological portion (Xv/0.8) of the settled biomass may be assumed. The modification of equation Eq. (8.4-4) to take sludge biodegradation into account yields:
Sludge = (SS - SSe - (Xv / 0.8) ⋅ kd) Q (8.34 lb/Mgal ⋅ (mg/L)) 365 day/yr Eq. (8.4-5) where: kd = Fraction of biosolids degraded (0.4 typical)
By assuming that the settled solids will compact to a 10% solids sludge (90% water, 10% suspended matter) with a specific gravity of 1.06, and by using the basin configuration, the reduction in depth due to sludge accumulation can be calculated. The reduction in depth will be equal to:
Sludge / (A (1.06) (0.10) 62.4 lb/ft3) = ft/yr Eq. (8.4-6)
where: A = Area of the settling basin, ft2
The solids should be cleaned out of the settling basin at intervals chosen such that the settling basin will have sufficient residence time even at maximum sludge levels, and the basin will have sufficient depth to control odors.
8.5 SAMPLE DESIGN PROBLEM
Design an aerated lagoon system capable of meeting the following effluent requirements:
COD = 80 mg/L
BOD5 = 20 mg/L
TSS = 55 mg/L
The following wastewater characterization data were obtained:
Average flow = 0.38 Mgal/d (1440 m3/d)
Design flow = 0.90 Mgal/d (3400 m3/d)
Design Influent COD = 300 mg/L
8.0 AERATED LAGOONS (Cont) Kinetic coefficients (Ym and b shown here taken from Table 6.1-1)
Ym = 0.5
b = 0.1 d-1
q (typical value) = 1.6 d-1 at 20°C Lagoon depth = 6.6 ft (2 m) Temperature Effects
Use Eq. 6.3-2 to correct for minimum temperature:
q25 = (1.6)[(1.08)(25-20)] q25 = 2.3 d-1 at 25°C Substrate Removal
Use Eq. 8.4-1 to solve for retention time in the aeration basin (HRT) based on COD concentrations:
S/So = 1/(1 + q HRT)
HRT = (300/80 - 1) / q
HRT = (3.75 - 1) / 2.3
HRT = 1.2 days
Solve for aeration basin volume: V = (HRT) Q = 1.2 days (0.90 Mgal/d) = 1.08 Mgal For an aeration basin with a depth of 6.6 ft, the surface area will be 21,900 ft2. Oxygen and Power Requirements
Influent COD in lb/d
COD = (So) Q (8.34 lb/Mgal ⋅ (mg/L))
COD = 300 mg/L ⋅ 0.90 Mgal/d (8.34 lb/Mgal ⋅ (mg/L)) = 2,250 lb/d Oxygen required for COD reduction
O2 = 1.2 ⋅ 2,250 lb/d = 2,700 lb O2/d HP required for oxygen transfer
HP = (2,700 lb O2/d) / (24 hr/d ⋅ 2 lb O2/bhp hr)
HP = 60 bhp
Check HP/Mgal to ensure it is adequate for complete mixing:
HP / V = 60 bhp / 1.08 Mgal = 56 bhp/Mgal Rule of thumb is 60 to 100 bhp/Mgal
Increasing the power to the aeration basin to 80 bhp meets this requirement (74 bhp/Mgal)
This 80 bhp requirement can be satisfied with 16 - 5 HP, 11 - 7.5 HP, 8 - 10 HP surface aerators, etc., depending on the size and depth of the aeration basin. Please see the aeration subsection of this DP for more details.
Sludge Accumulation
Calculate the mixed liquor volatile suspended solids:
Xv = ((So - S) Ym ) / (1 + b HRT) Xv = ((220 mg/L) 0.5) / (1 + 0.1)
Xv = 100 mg/L
Calculate the suspended solids in the aeration section effluent:
SS = SSo + Xv / 0.8
SS = 80 mg/L + (100 / 0.8) mg/L
SS = 205 mg/L
8.0 AERATED LAGOONS (Cont)
Assuming 40% of the biosolids in the settled sludge will degrade, calculate sludge buildup in the settling basin:
Sludge = (SS - SSe - (Xv/0.8) ⋅ kd) Q (8.34 lb/Mgal ⋅ (mg/L)) 365 day/yr
Sludge = (205 - 55 - (100/0.8) ⋅ 0.4) 0.90 Mgal/d (8.34 lb/Mgal ⋅ (mg/L)) 365 day/yr Sludge = 274,000 lb/year
Choose a retention time for the design flow rate of 0.9 Mgal/d:
5 days will be chosen in order to provide settling and minimize algal growth. This, along with a basin depth of 6.6 ft (2 m), defines a volume of 4.5 Mgal (602,000 ft3) and a surface area of 91,000 ft2.
Calculate reduction in settling basin depth:
Sludge / (A (1.06) (0.10) 62.4 lb/ft3)
(274,000 lb/yr) / (91,000 ft2 (1.06) (0.10) 62.4 lb/ft3) = 0.45 ft/yr 8.6 OPERATING STRATEGIES AND ENHANCEMENTS Oxygen Transfer
Older lagoon systems often used photosynthesis and surface contact with the atmosphere to provide the necessary oxygen for treatment. As loads to the wastewater treatment system increase due to process expansions, additional aeration may be needed to meet effluent discharge limits. The calculations shown above may be used to determine oxygen requirements for proper treatment and to determine aerator power requirements for oxygen transfer and complete mixing.
Caution is advised when adding aeration to an existing pond. The relatively shallow depth in most ponds is usually not recommended for a diffused aeration system. In addition, depending on the HP needed, a surface mechanical aerator (most commonly used in lagoon systems) may require a lagoon depth of 6.6 ft (2 m) or greater to avoid scouring the bottom of the basin. Please refer to Figure 4.2-1 of DP XIX-A6 for more information on choosing an aeration system.
Short Circuiting
Poor oxygen transfer may also be caused by short circuiting, a common problem in lagoon systems. When this occurs, the actual volume in the lagoon is not used effectively, due to poor pond configuration and/or temperature stratification, resulting in poor treatment in the “dead" zones. Other effects include ineffective use of aeration equipment, promotion of algal growth in dead zones, and surface scum accumulation.
The most effective way of correcting short circuiting is the use of baffles. Various materials have been used for baffle walls including fiberglass, PVC, and synthetic rubber. In addition, floating baffles are available from a number of manufacturers.
These baffles are supported by a continuous float at the top and a chain weight at the bottom. Baffles can be held in place by anchors located at the lagoon dikes. Floating baffles are especially useful for situations where upgrades to existing facilities, such as the addition of surface aerators, are to be made in the future.
Dye or tracer testing can be used to evaluate the severity of short circuiting. It is recommended that EMRE Environmental Specialists be consulted for proper methods and to provide assistance in planning a test program.
Algae and Suspended Solids Control
Extended retention times in the settling basin can promote algal growth, which can contribute to effluent TSS and BOD levels.
Although the growth rate is site specific, depending on available nutrients and sunlight, algae can usually be controlled by limiting the retention time in the settling basin to within 2 - 10 days.
High TSS levels can also be the result of inadequate settling basin volume. If this is the case, sections can be adjusted using floating baffles and/or by repositioning aerators.
Sludge Buildup and Removal
Settled sludge will need to be removed from the lagoon occasionally. A dredge or vacuum truck can be used for this purpose.
The dredged material can then be sent through the normal dewatering and disposal process. One disposal option, if feasible, is the mixing of dewatered biomass with oily sludge prior to landfarming. The biomass provides nutrients and aids in soil conditioning.
Biomass Return Options
With stricter effluent limits, it may become difficult for sites with lagoons to consistently meet these regulations without building
8.0 AERATED LAGOONS (Cont)
As part of the Code 500 Environmental R&D program, EMRE has investigated improvements in lagoon efficiency by increasing the concentration of biomass in the aeration zone. This increase in biomass should result in an improvement in biodegradation rates and a decrease in contaminant levels. One conceptual design which would be low cost, low maintenance, and easy to install is the suspended baffle design. In this system, water leaving the aeration zone would flow through a “race track"
configuration of suspended baffles. Solids settling below the baffles would then be pumped back to the aeration zone.
Recycling biomass may also improve nitrification by increasing system SRT (SRT greater than 20 days is usually required for nitrification), providing additional time for growth of the specialized microorganisms required for ammonia removal.
Biomass return for aerated lagoons remains a conceptual technology. It has not been field tested and cannot be recommended for full scale use without further investigation. Sites considering replacement of an aerated lagoon system with a more expensive activated sludge plant should consult EMRE Environmental Specialists for additional information on this upgrade option.
9.0 AEROBIC ATTACHED GROWTH