Transactions on Ecology and the Environment vol 26, © 1999 WIT Press, www.witpress.com, ISSN 1743-3541

Common problems in wastewater treatment plants function Gorana Cosic Flajsig Croatian Water-Zagreb 7WWZ^e6,6%^a(/a ^0X^220, Croaf/a Bojan Zmaic Civil Engineering Institute - Zagreb 10000 Zagreb, J.Rakuse /, Croatia EMail: [email protected]

Abstract Generally speaking, wastewater treatment plants are constructed to protect water environment from excessive overloading with different kinds of pollutants. So they have to fulfill the desired effluent standards. Often in practice, the results are not in accordance with design parameters expected from the project. Some common reasons for malfunction will be illustrated by several examples. There may be problems with the sewage system, design of plants technical equipment, hydraulics and of course with the maintenance connected with undertraining of personnel. Frequently, simple improvements in sewage and plant system technology and adequate personal training can be answer to wastewater treatment problems, instead of costly reconstruction. Introduction Water pollution is becoming a serious restrictive factor for sustainable development of any society. On weak economies of numerous transition or other developing countries, water quality management is one of the heaviest burdens. Part of the problems lais in planning grandiose, perfect, but unattainable programs, which are usually postponed. Worst of all is that realistic step by step solutions were neglected, so that even feasible projects were not realized. Construction of waste water treatment plants (WWTP), is only one segment (and frequently not the most expensive) of all complex measures that must be realized in the frame of water pollution control planning. Stormwater drainage, sewers reconstruction, sludge disposal and regulation of recipient sections are frequently much more complicated and of course costly. Already, the fact that

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in majority of settlements only about half of the population is connected to sewer systems (except in some greater cities), show the size of the task. Relatively small amounts of available money that have been invested in environmental protection in the past, did not gave expected results. The worst is, it seems, that we are not learning enough on basis of our own mistakes. Maybe it would be useful to reexamine the past policy, with the intention to spend money in the optimal way. At the end of 60's water quality management became an important task involved with the actual Water act. Water quality objectives, classification and categorization of surface waters based on US Water quality criteria, and strict time-limits for construction of wastewater treatment plants, seems a good base for solving all water pollution problems. UNDP Study for regulation of Sava river basin (1973), included water quality management as a very important problem. Consultant firms made a very detail review of existing pollution sources and calculated permissible BOD loads at appropriate minimal flows. Based on the mentioned strict legislation, money needed for investment in waste water treatment till year 2000, was estimated to be much higher than investment in flood protection, irrigation, erosion, navigation or hydro power stations. At that time the flood protection was extremely important because of serious material and people losses. Politics and experts (mainly from water work constructions) decided to postpone water quality protection measures for better days. So, the water quality management became a second rate activity. Attention paid to personal education and organizational development in services connected with water quality management was almost insignificant. Latter, in the 80's, the economical situation was going down, and all improvements in legislation and organization did not reach expected results. The problems were solved individually, from situation to situation, depending mainly on the influence of local politics and interest of powerful groups connected with designers, constructors and manufactures of equipment. Now, less than 10 % of treated wastewaters (if all the constructed treatment plants are working), is not a good result. However, a lot of high quality water resources (Fig.l.) and only few heavy polluting industries, still make Croatia a relatively clean European country, with a few "black" spots in the vicinity of some great urban centers. This lack of the problems connected with water resources is also one of the reasons why the water quality management in Croatia was not treated more stringently. With the new Water Act (1990), accented in the earlier introduced "polluter pay" principle, an economical base for realizing good water quality management is created. The war and the economical situation slowed down the realization of this Act. For its "reviving" it is necessary to convince all responsible factors in complexity of the problem, and the need for realistic steps according to the possibilities of our society . Actual plans supported by foreign "favorable" credits could solve only a part of the problem. They could

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be also a source for repetition of all the mentioned mistakes, with the burden which is too high for the actual living standard of our population. Besides this general review of influences on water quality management results, there are of course a lot of common technical and organizational problems in waste water treatment practice, more or less common in the whole world, which are the real subject of this report. Malfunction in waste water treatment plants In Croatia there are about sixty WWTPs between 2.000 and 400.000 PE (Fig.2.). But only about 20 have complete secondary treatment. The majority of these WWTPs do not satisfy the expected effects of treatment. During works on control, research and reconstruction of these treatment plants many different reasons for malfunction were found. This could be classified in several groups. Basic data In planning and designing of WWTPs basic data are generally of poor quality. Usually the main source are unrealistic urbanistic plans for development in combination with theoretical calculations based on tables for water consumption rates, runoff coefficients, population equivalents, etc., taken frequently from the out of date literature. Measured parameters of quantity and quality of waste water are used in very few situations. Justification of resulting excessive dimensions and volumes is based on poor "available data" and "factor of safety". Lack of experience The other problem is the obvious lack of practical experience, visible in beginner mistakes with wastewater flow, pumping, inlet and outlet hydraulics, detention time, aeration and sludge treatment. Lack of knowledge is reflected in frequent copying of foreign solutions and old projects, without taking in account local situations and other important influences. Very few constructed WWTPs and lack of systematic specialist education scarcely can legitimate such mistakes. Conception of sewer system Construction of the sewers in the past was mainly purposed for as faster as possible drainage of all present waters to the nearest recipient. With necessity of treating waste waters this approach is completely changed. W^WTP influenced the choice of combined or separate systems,, diminishing of drainage intensity, stormwater overflows, retention tanks, inline treating etc. Of course this is connected with costly reconstructions. Frequently this reconstruction step is omitted. In most cases one of the serious problems which influences the function of WWTPs is the sewer system which is not adapted for selected treatment (sometimes even not completely constructed). Usually designing processes for sewers and WWTPs are completely separated, done by different organizations, without necessary coordination and cooperation.

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Low quality of sewer system The quality of the constructed sewers is very different from place to place, and also dependent as the time when it was constructed. Frequently the channels are not water-tight, and drainage of underground or stormwater is a common problem. Obligation for water-tight investigation of new constructed sewers is improving this situation considerably. Insufficient slope of usually overdimensioned sewers connected with our general practice of garbage and other seatleable materials dumping into sewage system create serious problems with sedimentation and anaerobic situation in sewers. Low maintenance of sewer system Inferior economic situation in communal organizations influenced the poor maintenance of sewers. Dredging and rinsing was more an exception than practice, usually done when the sewers are plugged. Lately, with the acquisition of modern equipment for maintenance, this situation is quite improved in numerous cities. Industrial pretreatment High rash loads are a frequent reason for disorder and decaying of biological processes in existing WWTPs. Usual source for such loads are connected polluters which usually take minimum care about their wastewater. It is not strange that industrial pretreatment is missing at all. On the other hand unreasonable installation of expensive equipment connected more with marketing interests than with research and practical experience reflects in the poor performance of such expensive pretreatments. Connected with the low maintaining level this is one of the weakest points in the existing water pollution control. Construction of WWTPs Lack of experience and low technical level of constructing firms leads to the numerous mistakes and faults. Tanks and basins are not sufficiently water-tight. Constructions are not made as precisely as they should be and mechanical equipment can not be fitted properly.

Equipment On one side the installed equipment is of low quality (sometimes second hand) and on the other, the unnecessary expensive, high level technology is forced, in spite of the low technical level of the personnel.

Maintenance In such situations maintenance is too difficult and expensive for the existing communal organizations. Usually, after opening of a new WWTP, everybody involved in design, construction and equipment supply loose the interest in it. Necessity of effective function and proper maintaining is left to the helpless, insufficiently educated operator stretched between the lack of directors understanding and indolence and poor discipline of the available personal.

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A good communication between the responsible staff is very important for proper function of WWTP. Personnel problems Usually it is not possible to get the best workers on WWTP. This "dirty business" is not properly valorized. Unqualified and problematic persons ( frequently alcoholics or aggressive people) are often placed in such tasks. Financial problems The problems connected with high expenses and payment are not solved on the proper way. Frequently the communal organizations are saving money on account of personnel, chemicals and energy in such a way that WWTP is working in one shift, without necessary analytical control with half of the aerators excluded and other similar "economizing measures". Examples Velika Gorica (35.000 PE) It is one of the oldest wastewater treatment plant (1974) for a rapidly growing settlement in the vicinity of Zagreb, originally designed by "Hydroinzenjering" from Ljubljana, as activated sludge treatment. In a typical step by step construction this was a lot of uneven solutions, which always needs some new additions. In any case, beside all usual problems this WWTP was working relatively good for many years, thanks mainly to great voluntarity of the operators. The main problem was sludge treatment. Constructed anaerobic digesters never gave expected results and little by little were neglected. New centrifuge for sludge dewatering could not cope with insufficiently treated sludge. Provisionally it is disposed in the Zagreb sewer system. Next problems arise after construction of great sewer systems which collects wastewaters from all small villages near great protected watershed of Zagreb. These long sewers with high capacity pumping stations, beside wastewater, drain partly also groundwater and stormwater, creating high hydraulic loads, which could completely disturb the function of biological treatment. After such malfunctions anaerobic situations appears downstream in a low flow recipient with terrible smell irritating the inhabitants of nearby settlements. The result was the construction of quite expensive long effluent pipelines to the river Sava. Anticipated WWTP reconstruction (90.000 PE), once again does not take care about the existing problems in the sewer system. Daruvar(25.000/^Typical oxidation ditch, constructed by Unioninvest from Sarajevo (Passavant license) in 1976. Without experienced operators proper function began not earlier than 1979, but from that time it was one of the best secondary treatment in Croatia.

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Activated sludge system used only a half of volume because the other aerated basin was used for aerobic stabilization of sludge. That sludge had very poor dewatering characteristics (probably influenced with brewery waste waters), so existing drying beds were inadequate. Surplus sludge was disposed in natural lagoons near the WWTP. In the meantime, the capacity of WWTP become insufficient. In 1984 three solutions were proposed. First, original second step of Unioninvest, next more sophisticated solutions in existing location and the third one, on the new location because the old was surrounded by a residential area. The last solution was accepted. Due to the lack of money for such removal, it was never realized. For that reason there was no any further investment in the existing WWTP. The next proposal for reconstruction was made in 1993, and was a typical marketing design with the offer for all the most sophisticated up-todate equipment. Of course, it was too expensive for the capabilities of that small city. After field and pilot plant examinations which are still in the course, a simple solution with only one new object, secondary settling basin, seems reasonable and enough for the next few years of functioning of this WWTP. Measured quantities of dryweather flow were less than one third of that proposed in previous design. Pilot plant termophile aerobic treatment of sludge gave excellent results, reducing the quantity and improving the dewatering capabilities. Improving of the aeration system is still under investigation. Buzet (25.000 PE) WWTP constructed for brewery and city of Buzet is the most sophisticated, originally constructed as two-step activated sludge process with flotation instead primary settling. It is a copy of similar Roedigers plant of the city Lech in Switzerland. From 1980 when it was finished the function was only formal with symbolic efficiency. The main reason was complete lack of cooperation between the brewery staff and the communal organization. Brewery didn't take care about their technological processes and pretreatment, so the rash loads of heavy polluted active biological material reach the sewer and treatment basins, creating settlement and nuisance in further processes. Uneducated personnel could not cope with such problems in very demanded wastewater treatment technology. Fine bubbles aeration was clogged very fast, and anaerobic situations prevail. Engaged experts tried to improve processes by stronger coarse bubble aeration, addition of nutrients (P and N), liofllizate bacteriums and changing the process in a one step system. After unsuccessful attempts new reconstruction was proposed, almost doubling existing capacity. After negative expertise revision, other try in 1996 with experienced operator from Daruvar was much better. Approach was completely different. First, the technology of the brewery was improved with excellent cooperation with all personnel involved. Existing pretreatment was made usable with simple changes. Sewage system was dredged and rinsed first time from the

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construction. All basins of the WWTP were emptied, cleaned from sediment and disinfected. Machine and electrical equipment was provisionally repaired. After a few weeks the effluent BOD concentration for the first time declined below 10 mgO2/l. But a new problem came up. When the brewery was not working with full capacity, during nights and weekends, and especially stormy weather, concentration of organic material in wastewater was extremely low, partly because of the notable inflow of ground and storm water in the sewer. Brewery temporary helped by control addition of boiled waste yeast, which extremely improved the quality of activated sludge. Fish schools at the outlet of the WWTP was the best prize for the operators effort: Conclusions Field investigations on diferrent existing WWTP discover a lot of mistakes and defects which seriously influenced its function. Some of the common reasons are the problems with the sewage system, design of plants technical equipment, hydraulics and, of course, maintenance connected with low technical level of the operators. Frequently instead of costly reconstruction, simple improvements in sewage and plant system technology and adequate personnel training can be an answer to wastewater treatment problems. Some of the conclusions based on such approach are exposed in the next part. Technical - WWTP is only a part of the system for transporting, treatment and disposition of waste water and sludge, and from that standpoint all those activities should be reciprocally accustomed. - optimal data should be collected on basis of field investigations and pilot tests - one of the basic principal should be the flexibility in operation - in working out optimal solutions for all processes, final disposition of sludge should not be forgotten General conclusion - solution of financing should be based on standard possibilities - especially the legislation should be applicable and in accordance with economical reality - step-by-step solutions in accordance with natural processes must obtain advantage ahead of rude commercial interests

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- responsibility of designers, constructors and equipment delivers should be exactly defined, legislatively and financially - education at all levels, about water quality control, with a lot of practical lessons and exercise should be the main and continues activity Finally it could be said that sanitary engineers connected with practical work on waste water treatment plants frequently turn in collision with theoretical hypothesis or bureaucracy measures. In this field the gap between theory, desire and practice is too wide. Practical engineers are scarcely involved in planning, creating legislation or influencing any important decision in mainly administratively organized environmental and water protection authorities. The role of such people in scientific work or education in the universities is not sufficient. Even in the process of designing the waste water treatment plants, practical experience is far less important than superior marketing interests established by influenced equipment manufacturers. So, it is not strange, that almost all realized investments in water protection suffered numerous technical and organizational mistakes. So, in all these activities, practical experience should be taken more seriously.

References:

1. Vodopija M. and Cosic-Flajsig G., Sewage system and WWTP function, Pro. 1st Croatian Conference on Waters, Dubrovnik-Croatia, 1995 2. Zmaic B., Drainage System as Physical, Chemical and Biological Reactor, Pro. 1st Croatian Conference on Waters, Dubrovnik-Croatia, 1995 3. Zmaic B. Investigations on WWTP of Velika Gorica, Report, IGH-Zagreb 1995

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