Team:Bielefeld-Germany/Sewage treatment plant

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Sewage Treatment

Contents

Approach

The development of a project from planning to application includes several steps. Conventional industrial processes have to be compared to the potential application. Furthermore the potential application has to be compatible to existing structures, because the construction of new structures is cost-expensive and productivity-sapping as well. Therefore we cooperated with two sewage treatment plants. To cover large-scale plant as well as smaller rural plants we visited the treatment plant Obere Lutte in Gütersloh with a population equivalent of over 300000 and the treatment plant Schloß Holte-Stukenbrock with a population equivalent of about 30000. We discussed with the plant managers Mr. Burbaum (‘’Obere Lutter’’) and Mr. Bülter (‘’Schloß Holte-Stukenbrock’’). Together we were looking for potential operational sites within the different stages of treatment. We were discussing about advantages, limitations and problems, that could arise. We would like to thank Mr. Burbaum and Mr. Bülter for all their great ideas, their inspiration and for all the time, they invested.

Introduction

Sewage is generated every day by everyone. Therefore efficient sewage treatment is absolutely necessary to guarantee the quality of surface water, ground water and of course of drinking water. However sewage is often heterogeneous and consists of liquids from toilets, kitchen, sinks as well as liquids from industrial processes and commercial use. Sewage treatment plants have to consist of various treatment stages. The general composition of a large-scale treatment plant is shown in the flow diagram in picture 1.

Functional principle

Schematic

Figure 1: Flow diagram of a large-scale sewage treatment plant, including three stages: primary stage to remove heavy and big solids, secondary stage to remove dissolved and suspended organic material using micro-organisms and tertiary stage to remove particulate material and to disinfect, taken and translated from wikipedia.

The large-scale sewage treatment plant “Obere Lutter” is located near Gütersloh and is equipped with three sewage stages. The different stages will be explained in the following section.

Figure 2: Plan view of the sewage treatment plant Obere Lutter, located in Gütersloh. Picture is generously provided by the water and sewage board Obere Lutter.

Primary treatment: mechanical stage

Figure 3: Mechanical stage of the sewage treatment plant Obere Lutter with 1: Rake, 2: Intake pumping station, 3: Grit chamber, 4: Primary clarifier. Picture is generously provided by the water and sewage board Obere Lutter.

The first step in the sewage treatment ist the retaining of rude solids with rakes. Grates directly behind the rakes hinder swimming solids. The intake pumping station transports the water to the grit chamber and the primary clarifier at a slightly higher level. The grit chamber is also used to remove fat. The trapeziform cross section and aeration with air support the sedimation of sand, although the organic solid remain in the water. The sediment is removed with special movable plates and after washing and drying the sand can be used in road building. The Primary clarifier is the last step in the mechanical cleaning and works with sedimation, too. In modern sewage treatmant plants primary clarifier are rarely used because they lower the concentration of easy biodegradable sustances, which are necessary for denitrification. The mechanical stage is also called primary treatment.


Secondary treatment: first biological stage

Figure 4: First biological stage of the sewage treatment plant Obere Lutter with 5: Activated sludge tank, 6: Intermediate clarifier, 7: Return sludge pump, 8: Precipitant reservoir. Picture is generously provided by the water and sewage board Obere Lutter.

In the first biological stage, the so called secondary treatment, the organical content of the sewage is considerably reduced by the use of microorganisms. Most organical content are carbon, nitrogen or phosphorous compounds. Depending on tank number and design nitrification and denitrification can take place simultaneously or intermittent. The intermediate clarifier is used to remove the activated sludge with the help of return sludge pumps. For phosphorus removal chemical precipitation can be used through addition of iron salts. Also salts can be added if pH adjustment is necessary.


Secondary treatment: second biological stage

Figure 5: Second biological stage of the sewage treatment plant Obere Lutter with 9: compensation tank, 10: pump station, 11: upstream dentrification tank, 12: Activated sludge tank, 13: Secondary clarifier / sedimentation tank. Picture is generously provided by the water and sewage board Obere Lutter.

In the second biological stage compensation tanks are used to retent surplus water if the flow rate gets too large. Before the waste water is directed into another denitrification tank it gets lifted by a pump station. So the water can flow in a free gradient through the second biological stage. The second denitrification upstream to the actived sludge tank works with microorganism to further degrade nitrate compounds to elementary nitrogen. The operation procedure of the second activated sludge tank is similar to the sludge tanks used in the primary biological stage, but the tanks differ in size and flow rate. Therefore dimensions of the surface aeration are adjusted. Calcium carbonate may be added to adjust the pH.


Tertiary treament: fourth purification stage

Figure 6: Fourth purification stage of the sewage treatment plant Obere Lutter with 14: fixed bed denitrification, 15: flocculation filtration, 16: tertiary / maturation pond. Picture is generously provided by the water and sewage board Obere Lutter.

The fourth purification stage or so called tertiary treatment provides a further improvement of the effluent quality. After having passed the maturation pond the water is discharged into the environment, for example in the sea, in rivers, lakes, ground, etc.) The fixed bed dentrification lowers the concentration of nitrogen compounds with the use of biological elimination. The fixed bed works as a artificial surface for microorganisms to grow. As carbon source methanol may be added. The fixed bed consists of expanded clay, for example. Supporting flint layer provide the stability.

Floccation filtration is used to eliminate soluble phosphate based compounds. Upstream addition of iron salts causes precipitation of the phosphates as insoluble compounds. The filtration is aerated, air and wastewater flow co-current through the filter. Attached microorganisms and aeration cause nitrification, but aeration is also needed to eliminate the added methanol. The most common floccation filtration uses Biolit (stone powder) as fixed bed. The use of activated carbon is an arising application because activated carbon also eliminates micro-contaminants like pharmaceutical used compounds. For more information about the common floccation filtration methods click The new innovation: immobilized laccases.


Sludge treatment: utilization and disposal

Figure 7: Sludge utilization and disposal stage of the sewage treatment plant Obere Lutter with 17: digestion tank. Picture is generously provided by the water and sewage board Obere Lutter.

The biggest waste product of a sewage treatment plant is the remaining sludge. It is part of the waste water, but is also generated in the actived sludge tanks. In wastewater treatment plants the sludge is continuously pumped into digestion tanks. Via anerobic digestion the remaining organic compounds are hydrolysed via hydrolysis, acidogenesis and methanogenesis to water, carbon dioxide, methan and hydrogen. The biogas can further be use in electricity generation or for heating purposes. The remaining sludge is thickened, pressed and afterwards disposed via burning. for example.



The new innovation: Immobilized Laccases

New potential applications always have to be compared to conventional industrial processes. Ideal preconditions for the implementation of a new application are existing structures for the use of the application as well as better performance compared to the conventional methods. The cost comparison is another important part of the implementation. In the following section we will therefore compare the conventional methods with our system.


Conventional floccation filtration with Biolit

Figure 8: Schematic drawing of a fixed bed flocculation filtration using Biolit, flushed with a bottom-up flow. Inflow of the feed waste water, process air, wash air and rinse water is shown. Discharge of clarified water and backwash waste water.Taken and translated from mikroverunreinigungen.de, generously provided by Mr. Burbaum.

The most commonly used system for fixed bed floccation filtration is a system using ‘’Biolit’’, a stone powder. The fixed bed is supported with flint layers. The process combines filtration with biological carbon reduction and denitrification. The system is further used for elimination of phosphate based compounds. The functional principle is based on attached microorganisms. Therefore aeration is required. Aromatic compounds such as estrogens and steroid hormones as well as most residual pharmaceutical substances cannot be eliminated. Furthermore the use of attached microorganisms results in free-floating cells, which need to be filtered before using the water as drinking water.


Floccation filtration with activated carbon

Figure 9: Schematic drawing of a fixed bed flocculation filtration using activated carbon, flushed with a bottom-up flow. Inflow of the feed waste water, process air, wash air and rinse water is shown. Discharge of clarified water and backwash waste water.Taken and translated from mikroverunreinigungen.de, generously provided by Mr. Burbaum.

Recently activated carbon was used to replace the ‘’Biolit’’. The functional principle of activated carbon is based on its properties for adsorption. By using activated carbon the concentration of chemicals as chlorine as well as hormones and pharmaceutical substances can be reduced. But the capacity for removing pollutants depends on the surface area of the carbon. The longer the activated carbon is used, the weaker the adsorption gets. If the occupancy rate is too high, the activated carbon has to be removed and replaced. Reactivation and regeneration of the carbon is very expensive. The most commonly used method is desorption under high temperatures (500 – 900 °C). Alternatively the carbon can be burnt, but the acquirement of activated carbon is expensive as well. Mr. Burbaum of the treatment plant in Gütersloh puts the costs for the use of activated carbon at up to 2300 $ per ton for new activated carbon and up to 1600 $ per ton for reactivated carbon. The resulting costs for a treatment plant with a population equivalent of 300K can be estimated at up to 500000 $ per year.


A new approach: Immobilized Laccases

Figure 9: Schematic drawing of a fixed bed flocculation filtration using immobilized laccases on silica beads, flushed with a bottom-up flow. Inflow of the feed waste water, process air, wash air and rinse water is shown. Discharge of clarified water and backwash waste water.Taken and translated from mikroverunreinigungen.de, generously provided by Mr. Burbaum.

After having visited both of the sewage treatment plants we discussed with Mr. Burbaum and Mr. Bülter about potential, already exisiting structures that could be used for degradation of aromatic compounds with immobilized laccases. The best suited locations are shown with red arrows in the figures 5 and 6. The activated sludge tank (figure 5) is equipped with a fine meshed filter, large particles stay in the tank. Therefore the beads need to have a diameter of 5 mm, at least. Consequently silica beads cannot be used. Besides silica beads would sediment very fast. Another problem is the heterogeneous microorganism population. Emerging formation of biofilms would not only cover the beads but also hydrolyse the immobilized laccases. As a result the enzyme activity would decrease fast.

The better possibility is a fixed bed flocculation filtration tank filled with beads instead of activated carbon or ‘’Biolit’’. Additional supporting flint layers would minimize the discharge. Flushing with a bottom-up flow also guarantees a long exposure time. Additional layers of activated carbon may further increase the range of compounds, that can be eliminated. Figure 10 shows a schematic drawing of a possible fixed bed tank with immobilized laccases. The intake of process air, wash air and rinse water offers the possibility to adjust physical properties for example the pH value to optimize the enzyme activity.


Before using the system, assessment research has to be done. Degradation products of various substrates have to be quantified. The variation of the enzyme activity has to be measured over time. Finally the optimal enyzme concentration and the best filling level of the tank have to adjusted. Finally only a cost estimation could compare our project to conventional methods of fixed bed tanks.

Expert evaluation

In our discussions with Mr. Burbaum and Mr. Bülter, we were encouraged that our project might be a realistic alternative to conventional systems of eliminating micro contaminants such as steroid hormones and pharmaceutical compounds. Mr. Burbaum agreed with us that the contamination of drinking, ground and surface water with micro contaminants is a problem, which is even getting worse. Only large-scale treatment plants can possibly build up structures to eliminate the compounds in systems like fixed bed tanks filled with activated carbon. In smaller rural treatment plants often only two treatment stages exist. Therefore no structures could possibly be used for treatment with activated carbon or even with immobilized laccases. Cost-intensive construction projects are needed to face to problem of increasing concentrations of micro contaminants. Even on governmental level official consider various possibilities of solving the problem. Another problem is the limit of detection when it comes to steroid hormones. Often the no effect level is way smaller then the detection limit, therefore the consequences of permanent exposure can only hardly be measured.

Sewage treatment plant: Schloß Holte-Stukenbrock

Figure 11: Schematic drawing of the sewage treatment plant of Schloß Holte-Stukenbrock. 1: Rake, 2: Grit chamber, 3: Primary clarifier, 4: Intermediate clarifier, 5: Activated sludge tank, 6: Precipitant reservoir, 7: digestion tank, 8: maturation pond. Picture is generously provided by Mr. Bülter.


The sewage treatment plant of Schloß Holte-Stukenbrock is a small rurar treatment plant with a population equivalent of 30000. Only few industrial waste water has to be treated. The plant is equipped with two treatment stages. Therefore an implementation

Conclusion



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