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
Where:
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
KT = K20 Q (T-20)
Where:
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:
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.
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:
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.
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,
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.
Summary
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.
We will help you further.Thanks.
Thanks for sharing!!
Kindly share stated figures i.e. Figure 1, 2, and 3, for better clarity.
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shakeel
Your presentation is a summary of the whole problem of all countries of the world. In Dakar or the temperature is higher than the one you describe there is the same problem.
I think you made a very big mistake by using the word processing -biologique- when all along your benefit you only speak technicality. A -biologique- treatment is carried out by a -biologique- element that is found in a sewage effluent biologic-which is not the case of sewerage effluents. They are -biochimique- and it can be no treatment - biologically.
How do you prove what I say?
Simply by producing mud. Sanitation -biologique- produces no mud, it eliminates all organic mass and mainly feces excrement. As these are -biologique- it promotes the elimination of other organic body masses. The treatment of sewage treatment plants, as you demonstrate so well in your presentation focuses on filtration and think even add to improve treatment. Any technical action deteriorates the characteristic -biologique- effluents.
Conclusion: You can not perform a biological treatment in the conditions you listed.
My first concern by developing organic cleansing that was the temperature. So the bacterial -biologique- job runs every degree of temperature, it just slows down in low temperature and increases in high temperature. So that is called -biodégradation- work can be done so you have to extend the transit time of the effluent treatment system at low temperature period and accelerate high temperature. Which of course is impossible to do on a sewage conditioned by the flow of wastewater to be treated. In high temperatures is the problem of putrefaction. An organic material in a liquid under a large temperature by quickly decaying and will methanised thoroughly.
From experience the increase in temperature with a biological effluent causes two obligations:
- That the medium is aerobic state as a whole and to all the places where they will find this is not the case in a wastewater treatment plant
- The transit time is accelerated.
I was able to conduct tests on a forced air adding micro-station. After a month analysis showed a slight decrease in processing performance. forced air destroys the characteristic - biologically in the oxidant. By a lucky chance I could confirm this by finding a solution to -biologique- following traces the treatment performance -biologique-.
In domestic sewage from the sewerage is dumped every day thousands of liters of cleaning products containing hazardous chemicals microphones. We must add all sanitizers and disinfectants for medical and para medical, destroying all the feature - biologically effluent. When added to the supply air is caused by a reaction chain of these chemical particles which further oxidize the medium.
The first rule to maintain a state -biologique- effluent is to eliminate the addition of chemicals. Of course, impossible to do in the collective principle. By implanting against the concept of a metropolitan CEBRE is possible to improve the conditions and slightly increase the purification performance of a wastewater treatment plant which is zero, close to zero from the perspective -biologique- .