A temporary, minimum cost solution to Prague's wastewater problems, called the intensification of the Prague Central Wastewater Treatment Plant (CWWTP), was put in operation in August 1997 and is now hitting most of its targets.
It has even hit some of the targets for the projected high-cost permanent solution with still lower contaminant limits set by the environmental authority and adopted by Government statute in 1992.
The project's ambitious goals were to enlarge the capacity of the biological part of the CWWTP with new plant located in the existing, very limited space at Cisarsky Island. A new regeneration tank for returned activated sludge was built in the remaining area of the WWTP, which allowed increased residence time for aerobic sludge and therefore achieved nitrification throughout the year.
New secondary settling tanks were built to improve hydraulic capacity of the WWTP and the aeration system in the activation tanks was renewed. Fine bubble aeration ensured sufficient oxygenation capacity for the increased amount of wastewater treated by the activated sludge process and for nitrification of ammonia nitrogen.
Compliance difficulties
The first year of the trial operation demonstrated that the hydraulic capacity of the secondary settling tanks was 7 m3.s-1 and thus all Prague wastewater has been treated by the activated sludge process since August 1997. Achieving the prescribed very low pollution limits in the effluent was difficult. The main reasons were, and still are, seasonal problems with sedimentation properties of activated sludge, denitrification of nitrified nitrogen in secondary settling tanks and low capacity of the sludge treatment system.
The consequences were frequent, very high concentration peaks of suspended solids (SS) and COD in the effluent. These peaks prevented building authorities giving final approval of the intensified WWTP. Achieving the very tough limits demanded by the environmental authority proved to be impossible and for these reasons the trial operation of the intensified CWWTP has been extended twice, each time by one year.
Unfortunately, the very good sedimentation properties of activated sludge anticipated by the designer, are seasonally worsened to a great extent by filamentous bulking caused by microorganisms Nocardia and Nostocoida.
This bulking caused mass overload of secondary settling tanks, rise of the sludge blanket level in the secondary settling tanks to outlet troughs and the necessity for the decrease of dry matter in mixed liquor. With lower sludge age, nitrification was impossible in the winter season.
Mixed liquor with concentration of nitrate nitrogen over 10 mg.l-1 flows into the secondary settling tanks in the course of nitrification. During the short contact time in the activation tank, microorganisms of activated sludge show high values of endogenous respiration.
Under these conditions intensive denitrification in the secondary settling tanks occurs and the gaseous nitrogen formed, floats suspended solids into the effluent. The situation is worsened by the inflow of considerably lower amounts of more concentrated wastewater than was anticipated by supplier and contractor of the project.
The operator is forced to run the plant on the very edge between the correct degree of nitrification and of carbonic contaminants entering the effluent. Insufficient capacity in the digesting tanks is caused not only by their small volume, but also by the intensive foaming of the digested sludge.
Problem solving
Two approaches were taken to solve the problem of nitrification under conditions of decreased amounts of activated sludge. The first was the improvement of the primary sedimentation effect by the reconstruction of primary sludge pumping and especially by dosing anion-active flocculant with ferric sulphate.
These measures brought a decrease of the mass load at activation stage and therefore the rise of sludge age even with low dry matter concentration in the activation tank. The second was running filtered water with high ammonia nitrogen content from the sludge treatment system into the regeneration tank, instead of leading it into the inlet of mechanically pretreated wastewater, thus using it to improve nitrification.
Improved primary sedimentation causes a change in the ratio of primary versus surplus activated sludge to the benefit of primary sludge. This way, sludge with a lower rate of filamentous microorganisms is dosed in the digesting tanks. These microorganisms, among other problems, support foaming of anaerobic reactors due to their hydrophobicity. At the same time, part of the ferric sulphate was added to the activation tank. This brought a decline of phosphorus concentration in treated water and probably contributed to reduction of released carbonic pollution.
Control and consultation
In order to control this kind of technology, it is necessary to have the highest possible amount of operational data immediately to hand. That is why the CWWTP was equipped with extensive control systems during trials. Many sensors and final control elements are plugged in the control system.
The operator has had calibrated mathematical model of the CWWTP's technology called HYDROMANTIS GPS-X since summer 2000. Searching of optimal operating strategy is first carried out on the model and then realized in full scale. Apart from this, the operator tested any improvement of sludge treatment capacity by digesting under thermophilic conditions.
Beginning in June 2000 pilot plant scale test of direct deamonification of filtered water from dewatering centrifuges by KALDNES technology, in cooperation with the German PURAC company, has been carried out.
During the three years of trial operation, top Czech and foreign specialists were consulted to contribute to solving the problems of Prague CWWTP. A team lead by Professor Dohanyos from the Prague Institute of Chemical Technology (PICT) played a significant part in the work on problems of the sludge treatment system.
Troubles specifically with the activated sludge part of the Prague CWWTP were tackled by the operator, which is Prazske vodovody a kanalizace a.s. (PVK a.s.), in consultation with Dr Novak from AQUA-CONTACT PRAHA v.o.s., Prof Grau and Prof Wanner from PICT PRAGUE and of course with employees of the designer and builder of the plant, HYDROPROJEKT PRAHA. Important technical and technological help was provided by Ing Cervenka from KEMIFLOC. Another significant contribution came in 1999, when expertise was made available by Prof Grau (PICT) in cooperation with Prof Mogens Henze from the Technical University of Denmark.
At the beginning of 2000 the director general of PVK a.s. appointed an expert committee for solving operational problems of the Prague CWWTP. This was composed of Prof Wanner, Prof Dohanyos, Dr Novak, Ing Kutil, Ing Sorm, Ing Motl, Ing Pospech and Ing Roskota. The work of this expert group brought several new drives to improve performance of the plant. The work of the specialists led to many changes of the technology and produced further benefits.
A significant break-through came in the second half of 2000, when stable operation of the technological throughput and significant improvement of effluent quality was achieved. Although full compliance with the limits concerning treated water quality was not attained, the present state is considerably better than before and it is probably the maximum that can be achieved with the existing technology. The greatest problems appear to be the concentration peaks that cannot be excluded from the data due to the CWWTP's consent based on Government Act 171/1992.
Conclusions
In quality testing of the treated wastewater from CWWTP Prague, for eight test parameters the latest results are within the limits for trial operation and for three parameters the results achieved are better even than the limits for permanent operation.
Further significant decrease of pollution released from Prague CWWTP can be achieved only by new investments in technology. This is the reason why the property administrator, firm Prazska vodohospodarska spoleenost a.s. prepared so called "Short term measures" and several more investment actions to achieve lowering concentrations of total inorganic nitrogen in the effluent below 20 mg.l-1 and further process stabilization.
These are especially the introduction of inoxic zones into the activated sludge process, dosing of the external substrate and chlorination of returned activated sludge. It also includes the building of equipment for cation active flocculant in mixed liquor before it enters secondary settling tanks to correct sedimentation properties of activated sludge and the remodelling of digesting tank agitation equipment.
