The sewage is collected in the vacuum station (Figure 7.3). From there, it is pumped into the equalization tank. This tank is designed for the equalization of the quantity of sewage over the day, so that the effluent of the plant can be kept steady.
This is as important as the separation of the rainwater, because the membrane area, is designed according to the maximum hydraulic load.
The next stage is a sedimentation tank (Figure 7.4a), where the solids are settled for separate treatment. The solids, which are up to 1–2% of the influent quantity, are treated in a high-load digester tempered to 37°C with a retention time of five to ten days.
To make the treatment more efficient, water is removed from the digestion process by a rotating disk filter.
The water with non-settleable solids flows from the sedimentation tank into a non-tempered, fully mixed anaerobic bioreactor. The mixed sludge is circulated over an external rotating disk filter (Figure 7.4b), which is continuously withdrawing the effluent from the bioreactor.
Figure 7.2 The concept DEUS 21.
Semi-centralisedurbanwatermanagement
The rotating disk filters are equipped with ceramic microfiltration membranes with a nominal pore diameter of 0.2 µm.
Fouling control is achieved by rotating the shaft, on which the filtration disks are fixed, creating a centrifugal force which causes the solids to flow off.
The treatment process splits up the wastewater into the solids-free filtrate, the biogas from the two anaerobic bioreactors, and the stabilized solids after anaerobic digestion.
The bioreactors were initially inoculated with anaerobic sludge from a sewage sludge digestion plant. Since for anaerobic treatment of wastewater a different composition of the anaerobic biocenosis is needed than for the digestion of sludge, the bioreactor for the wastewater treatment had to be started up slowly. In November 2009, pellet sludge from a plant for the treatment of wastewater from the production of fruit juice was added, leading to a more stable operation immediately due to the higher concentration of methanogenic microorganisms.
Figure 7.3 Scheme of the anaerobic MBR process in Knittlingen.
Figure 7.4 View of the wastewater treatment: left photographs, from left to right: equalization tank, sedimentation tank, bioreactor (water) and rotating disk filters at the right photographs.
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Water quality control and monitoring
Samples of the influent of the bioreactor and the effluent of the filters were taken about twice a week during the plant operation.
The measured concentrations of COD (chemical oxygen demand) and nutrients are presented in Table 7.1 for the period from November 2009 until July 2010. The temperature in the bioreactor ranged from 12.8 to 26.5°C, due to seasonal changes. High COD values above 155 mg/L occurred only in winter when the temperature in the bioreactor was below 14°C. This can be explained by a reduced activity of the methanogens at low temperatures. The nutrients nitrogen and phosphorous are not eliminated by the anaerobic treatment process and leave the system dissolved in the effluent, mainly as ammonium and phosphate.
Samples for the analysis of microbiological parameters were taken on three different days during operation of the plant, in the effluent of each of the four filter modules. Average values for total coliforms andE. coliare presented in Table 7.1. It can be seen that most bacteria are retained by the membrane, although re-contamination of the effluent is an issue, due to the relatively high concentrations of nutrients and COD. Experiments showed that the real cut-off is far below the nominal pore size of 0.2 µm, which makes it likely that most viruses, which tend to attach to solids anyway, are retained by the membrane as well.
In the period from January 2010 until July 2010, the average influent flow of the bioreactor for wastewater treatment was 5.7 m3/d, leading to an average hydraulic retention time of 43 hours (minimum 26 hours) and an average organic loading rate of 0.51 g COD/L·d (maximum 0.84 g COD/L.d). The average elimination rate of COD was 85%. The eliminated COD was transformed into biogas, most of it leaving the reactor over the gas meter, while about 10% of the produced biogas could be found dissolved in the effluent. By air stripping, it could be removed from the effluent and added to incineration.
Main challenges for operation
After securing the effluent quality, the achievable flux of the membrane filtration is critical for the sustainability of the process.
The sustainable flux according to Bacchinet al.(2006) depends mainly on the characteristics of the sludge. The higher the solids concentration of the sludge, the lower is the sustainable flux. For the biological elimination of the COD, on the other hand, a certain concentration of microorganisms is required. Temperature of the sludge is important as well. With decreasing temperature, the activity of the microorganisms decreases, requiring a higher concentration of microorganisms.
At the same time, the dynamic viscosity of the sludge increases with decreasing temperatures, which has a negative impact on the attainable fluxes.
Measures to increase the attainable flux are the rotation of the filter disks, regular back-flushing with filtrate and chemical maintenance cleaning. It could be shown that with higher rotation velocities the sustainable flux increases, at least up to rotation velocities between 300 and 350 rpm (Mohr, 2011). Back-flushing with filtrate should be automatically carried out every few hours for some minutes, while the interval for chemical maintenance cleaning of the rotating disk filter with ceramic membranes is about a year (Zechet al.2011).
The demonstration plant in Knittlingen was operating with a VSS (volatile suspended solids) of 20 to 25 g/L. With a rotation velocity of 300 rpm and a temperature between 20 and 25°C, sustainable fluxes of 13 to 14 L/m2· h have been determined. At temperatures between 10 and 15°C, the sustainable flux is likely to be around 12 L/m2 · h. There are many options to optimize the sustainable flux, though. If, for example, the ratio of methanogens–being responsible for the limiting step in the degradation chain–in the sludge can be increased, the VSS of the sludge can be decreased, leading to a
Table 7.1 Water quality of influent and effluent of the demonstration plant between November 2009 and July 2010.
higher sustainable flux. Other options are optimizing the automatic back-flushing and increasing the rotation velocity even more.
A major challenge in the operation of the plant in Knittlingen was the start-up, as there was no adequate inoculum available.
With the sludge that has grown in the bioreactor during operation, a new plant could be started up in less than a month. Even over a period of several months without operation, the sludge does not lose its activity, as reported by Lettingaet al.(2001).
If the nutrients cannot be recycled together with the purified water for irrigation, they must be eliminated or, better, regained out of the effluent. Depending on the climate conditions, outside the growing periods or during rainy seasons, there might be months during which the water cannot be reused in agriculture. Phosphate can be eliminated by precipitation with iron or aluminum salts. This leads to compounds where phosphorous is bound in a way that a further utilization is not economical any more. If phosphate can be precipitated as magnesium-ammonium-phosphate (MAP, struvite) from the effluent, it can be utilized as a fertilizer.
Even if most of the phosphate is precipitated as MAP, only a fraction of the ammonium of the effluent is eliminated this way (see Table 7.1). For the elimination of nitrogen by a nitrification/denitrification process, as it is usually applied in aerobic sewage treatment, there is not enough carbon left in the effluent of the plant. A solution could be a by-pass of a part of the influent. This way, the nitrogen can be eliminated, but will not be recycled. Recycling of the nitrogen as ammonium sulphate is possible by applying an process with zeolite as ion exchanger and ammonia stripping (Aiyuk et al. 2004), which has been successfully tested with the effluent of the plant in Knittlingen.
7.3 POTENTIAL WATER REUSE APPLICATIONS
The reuse of the AnMBR effluent in agriculture enables to valorise both water and nutrients. It also saves the effort of operating an additional process for the removal of the nutrients. As the effluent passes a microfiltration membrane, it is free of helminth eggs and parasites. Even though the effluent could contain bacteria due to regrowth, bacteria originating from the wastewater are retained by the membrane as well, preventing the spreading of diseases through the wastewater. If operating with a microfiltration membrane, it is not sure that all viruses are retained, as they are smaller than the pore diameter. Anyway, the secondary layer on the membrane and the fact that viruses tend to adsorb to particles make it probable that most viruses are held back as well. To increase the chance to keep the effluent free of viruses, ultrafiltration membranes with smaller pore diameters can be used instead.
Compared to the effluents of modern aerobic wastewater treatment plants, the COD of the AnMBR process is relatively high. This can cause increased microbial re-growth in distribution networks. If the effluent is reused, blocking of irrigation pipes by biofilms has to be prevented by regular flushing. When the effluent has to be stored for more than few days, odour has to be controlled or prevented by aeration. If the demand for irrigation should be covered only by the effluent of the depicted process, the risk of over-fertilization would be very high due to the high concentrations of nitrogen. As the amount of the effluent that can be provided to an area is limited by the maximum nitrogen demand of the plants, the demands for water and phosphorous could not be covered. This means that water and phosphorous have to be added to plants irrigated with an AnMBR effluent.
Another potential disadvantage of irrigation with wastewater, independently of the treatment process, is the high concentration of dissolved salts that can cause damages to sensitive plants, the soil, and the groundwater. The amount of salts in the water can be reduced by supplying a relatively soft drinking water, but a certain amount of salts will be added to the water by usage anyway. To prevent evaporation, which would increase the risk of salinization even more, subsurface irrigation systems should be used. The addition of rainwater collected separately from the effluent of the wastewater treatment plant from the roofs of the houses would decrease the concentration of salts and of nitrogen, decreasing the risks of salinization and over-fertilization.
Risks by heavy metals and organic trace contaminants cannot be excluded when reusing wastewater, although substances adsorbing to particles are retained by the membrane in the depicted process. In the semi-centralised approach, the addition of industrial wastewaters carrying higher concentrations of these substances can be prevented more easily and inhabitants can be informed about the importance of not adding certain substances to the sewage.
7.4 ECONOMICS OF WATER REUSE
Project funding and costs
The costs for investment and operation of the research plant cannot be compared to those of plants operating under economic conditions, because the objectives are totally different. Still, factors influencing the costs of the studied process can be identified. As for any MBR-process, the specific amount of water to be treated should be kept small, because costs
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associated with the membranes rise linearly with the amount of water. This means, intelligent water management concepts should keep the rainwater away from the wastewater and help minimize the per capita water consumption.
Concerning the investment costs, a single semi-centralised plant will certainly be specifically more expensive than a centralised plant due to scale effects. The effect of scale can be realized for semi-centralised plants if a large number of them are constructed in a region. Currently, the rotating disk filters used in the research plant are still prototypes. Further development is going on, with the objective of having an economically feasible filter ready in 2012.
The optimization of the wastewater treatment process can also have a share in cost reduction. Increasing the sustainable flux to values around 20 L/m2· h seems realistic. This has again an effect on the specific membrane area, influencing both capital and operation costs.
The geographic location also influences the economics of the DEUS 21 concept. If there is a demand for heat, the energy that can be won from the biogas can be utilized to a large extent, resulting in savings of other energy sources. The digestion of the solids in the separate reactor achieved degradation rates of 60 to 70% of the VSS. Assuming that all households add their kitchen wastes to the wastewater, a biogas production of at least 80 liter per inhabitant and day (80 L/cap·d) can be expected from the digestion of the liquid and the solid organics together. This amount of biogas has an energy potential of about 200 kWh per inhabitant and year. The same accounts to the utilization of the water and the nutrients phosphorus and nitrogen. If there is a demand close to the treatment plant, these savings can be used to decrease the operating costs of the process. Moreover, a process to eliminate the nutrients becomes unnecessary, if a utilization throughout the year (e.g. with greenhouses) or the storage of the effluents is possible.
Because of the low growth rates of the anaerobic microorganisms, the amount of excess sludge in the AnMBR process is only 10 to 20% of those of secondary sludge produced by the aerobic activated sludge process (Mohr, 2011). This saves costs for the treatment, transport and disposal of sludge. Labour costs are low in the DEUS 21 concept, because the wastewater treatment plant is fully automated and can be remote controlled from any computer that has connection to the internet.
Therefore, few experts are able to control a large number of automated, semi-centralised treatment plants, while once or twice a week a trained person has to physically visit the plants to check and if necessary maintain the aggregates, like pumps and filters.