IMPROVE PERFORMANCE OF BIOLOGICAL WASTEWATER TREATMENT PLANTS IN COLD CLIMATE
The primary element in the majority of direct discharge industrial wastewater treatment facilities is the biological treatment plant. In biological treatment the organic pollutants in the waste are biodegraded and converted to carbon dioxide and water. The performance of the organisms responsible for this degradation is significantly impacted by changes in wastewater temperature.
An understanding of the kinetic relationships of biodegradation can assist the wastewater treatment plant operator in overcoming the problems which are experienced in the operation of biological treatment facilities during cold weather conditions. kinetic relationships of biological treatment. Eckenfelder (1) hasdescribed the kinetics of oxidation as:
(So - Se ) / X t k = Se / So
So = Influent BOD (mg/L)
Se = Effluent BOD (mg/L)
X = MLVSS (mg/L)
t = detention time (days)
K = Reaction Rate Coefficient (day-‘)
The factor K is the kinetic rate coefficient and has been found to be related to temperature.
This variation with temperature can be described by the relationship
Q = temperature coefficient (typically 1.02 to 1.08)
T = Temperature ("C)
Using the type of organisms typically found in municipal and industrial wastewater treatment facilities, the optimum treatment plant performance is found in the temperature range of 20°C to 35°C. At temperatures above 35°C we begin to see deterioration in the biological floc as the facilities begin to experience problems with settling and solids - liquid separation.
However, the most common temperature problem is related to cold temperature and winter operation. Under these conditions there is reduced biological activity which can result in a deterioration of treatment quality and exceeds the permit limitations. As a "rule of thumb", the rate of biological activity and treatment doubles with each 10°C rise in wastewater temperature or, conversely, is cut in half with each 10°C drop. At wastewater temperatures below 5°C biological treatment activity drops to near zero. To minimize problems with biological wastewater treatment, the wastewater temperature in the biological processes should be maintained above 10°C throughout the year.
To assist the engineers and operations in evaluating temperature control modifications, it is useful to employ a temperature model. There are a number of temperature models which are available to predict the basin temperatures. We have found these models to be effective in evaluating basin temperature and implementing control technologies. Some of the primary the factors which affect basin temperature are shown in Figure 1.
There are a number of alternatives available to engineers and operators to overcome problems related to cold temperature operation. Our experience has shown that these controls can be categorized as follows:
- a) Increase Wastewater Temperature
- b) Increase Biological Activity
- c) Reduce Waste loading
Specific examples of these controls are presented in the following sections.
Increase Aeration Basin Temperature
There are a number of approaches which are available to maintain wastewater temperatures during treatment. These include reduction of heat losses during treatment and providing additional heat.
- Control of Heat Loss
Cooling or heat loses in a wastewater treatment plant occurs because of heat transfer through the bottoms, wall and top surfaces of treatment tanks and basins. In the case of walls and floors of these tanks, the heat transfer is by conductive heat flux into the surrounding soil and/or atmosphere. For the water surface these losses occur through radiation out of the water surface, evaporation and conduction transfer. The majority of heat loss and temperature drop in biological wastewater treatment plants is in aeration basins and trickling filters. The primary mechanism for heat loss through conductive and evaporative losses is aeration. Mechanical surface aeration results in significantly greater heat loss than diffused aeration. Figure 1 presents a schematic of the mechanisms for heat loss. To reduce this temperature loss there are several aeration approaches. These include:
- Adding mixers to the aeration basin. During winter operation there are many times when there is more than adequate aeration for oxygen transfer, but in order to provide adequate mixing requirements all aerators must be maintained in operation.
There are mixing devices which can provide up to a 90% reduction in power requirements from surface aerators and which can provide the necessary energy for mixing of the basin while minimizing surface agitation and heat loss. This allows a reduction in the number of surface aerators in operation and reduces temperature loss. Figure 2 shows the improvement in system performance with the addition of mixers.
- Use a low profile modification on the surface aerators. In this case the cross sectional area of surface sprays is reduced so that there is less evaporation and temperature loss.
- Reduced aeration during periods of extreme cold and significant wind. It has been our experience that the major temperature loss occurs on cold winter nights. Under these conditions it may be advantages to cycle all or a portion of the aerators off during nightly cold periods and on during warmer daytime temperatures. Although there would be some reduction in treatment efficiency during this compromised mode of operation, the maintenance of a higher overall temperature in the basin and allows better overall biological treatment performance.
- Modification of the aeration system from surface aeration to diffused aeration. Substitution of diffused aeration is another approach which can reduce temperature loss. Diffused aeration systems can provide the same level of aeration and mixing with a significant reduction in the heat loss caused by mechanical aeration through the surface spray. Diffused aeration systems will run warmer than a mechanical aeration system. The improvement in an industrial treatment system which was converted to diffuse from surface aeration is shown in Figure 3.
- Insulation of tank wells. Typically most aeration basins have been in-ground systems and the ground provides very good insulation. However, a number of newer basins are being constructed in above-ground metal tanks. Although this is an efficient system and is being recommended for some industries by regulatory authorities, it reduces the insulation value provided by soils surrounding below grade tanks and basins. We have found several cases where it is essential to insulate the walls of both the aeration basin and equalization basins which may occur ahead of the aeration basins in order to reduce heat loss. Recent modeling which we have conducted has indicated that this can provide up to an 8°C increase under winter conditions.
- Install tank covers. Most wastewater treatment tanks are open-topped and exposed to the atmosphere. Installation of a fixed or floating cover on a wastewater tank can hinder observation of a treatment process or create potential human entry problems. However, installation of an insulated or non-insulated cover significantly reduces heat loss and raises the equilibrium temperature of aeration basins and other The capital cost of the cover installation should be compared to the annual cost of adding heat to the wastewater to determine the correct choice. This approach was recently used on a package plant and resulted in a significant improvement in winter operation.
- Elimination of trickling filters. Trickling filters, although simple and reliable methods of reducing BOD, are sources of large heat losses in wastewater treatment plants. Cooling of wastewaters during passage through the filter during cold weather may significantly reduce the rate of biological treatment activity in both the trickling filter and downstream activated sludge processes. This reduction in temperature may offset the potential BOD removals available from a trickling filter under winter conditions.
- Heating the Basin
Another approach is to add heat to the basin. The injection of steam or hot water either into the aeration basin or into the raw waste ahead of the aeration basin can increase the basin temperature. A steam injector has been utilized for this application. Experience in treatment of a refinery wastewater has shown that steam addition has significantly improved the wastewater treatment plant performance so that nitrification could occur on a year round basis provided that under cold temperature conditions steam is injected to maintain aeration basin temperature. Recently, in a pulp and paper application, the approach selected to control winter temperature was to make changes in the mill. It was determined that the bleach plant filtrate was using cold river water and that this was discharged to the aeration basin. By implementing a bleach plant recycle system, 1 MGD of river water was eliminated from the discharge which represented approximately 10% of the mill flow and there was a 3°C increase in the winter discharge temperatures. This produced a significant improvement in the overall treatment plant performance.
Increase in Biological Activity
Normally, there is adequate activity of the organisms to produce the desired treatment level without the need to increase biological activity. However there are some options for increasing biological activity.
For example, in many cases, we have found the kinetics to be nutrient limited. In these cases, it may be possible to increase the reaction rate,
- By adding nutrients. In this case, the increase in reaction rate because of nutrients will more than compensate for the reduction in reaction rate because of cold temperature and the system can perform under both winter and summer conditions. There have been a number of treatment plants which have found it necessary to increase nutrient addition under winter conditions.
one of the factors which relates to biodegradation is the MLVSS concentration. To overcome a reduction in the reaction rate in the winter, it is possible to increase the number of microorganisms. This will help maintain the overall removal rate and thus maintain treatment plant performance. In some cases where mixing in aeration basins or lagoons is marginal, an increase in biological degradation rate can be accomplished by increasing the effective MLVSS through intense mixing and exposure of greater numbers of organisms to the wastewater by preventing bottom deposition of organisms.
Reduction of Waste load
Another approach in bringing the treatment plant into compliance under cold temperature conditions is to reduce the waste load to the biological system. One mechanism for achieving this objective would be the supplemental addition of powdered activated carbon to the aeration basin under cold temperature conditions. Under these conditions, it would be possible to adsorb some of the organics rather than biodegrade these organics. The adsorbed organics could then be removed and disposed with the sludge handling system. This would reduce the load on the biological system and allow more consistent compliance under winter operations. This can be especially effective for refractive, moderately biodegradable or toxic organic compounds.
Another related approach to reduce waste loads would be to pretreat the waste under winter conditions. For example, if there was a primary treatment system such as coagulation or dissolved air floatation, it may be possible to increase coagulant dosages under winter operations and break out more of the organic load. This would then reduce the loading to the biological treatment process and allow more consistent winter operation. An additional approach for winter operation would be to hold back on the discharge of concentrated waste under cold temperature conditions. Many plants may find that a good portion of the waste load is in a concentrated low flow discharge. Therefore, in this case this waste could be held up in an equalization basin or spill pond and then recycled to the treatment plant as weather conditions and treated wastewaters warm up. This type of approach has been used at a number of chemical plants and pulp and paper mills and can be a cost effective approach toward reducing and controlling waste loads under winter conditions.
In summary temperature plays a major role in the performance of biological treatment plants. However, there a number of approaches available to the engineer and operators which can be Utilized to control temperature. If one is involved in design of a new treatment plant then the full range of options should be considered or if temperature problems are occurring with an existing plant, the options which are applicable to that plant should be examined. These approaches will allow the most cost-effective operation in compliance with winter operations.
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