Waste stabilization ponds in Greece: case studies and perspectives

Konstantinos Tsagarakis

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All waste stabilization pond (WSP) systems in Greece are presented, including primary and secondary data and qualitative parameters concerning their design and operation. Operation and maintenance practices are evaluated and improvements are recommended. Existing full-scale systems show that when they are properly designed and maintained, this can be a suitable and viable option for small communities. On the other hand, systems that receive minor or no maintenance have been proved to operate moderately, while conventional systems of the same size would have failed. The main drawback of the application of WSP systems is the lack of appropriate engineering and political will.

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Table 1. Qualitative characteristics of the wastewater at different treatment stages, for air temperatures ibove 10°C (Parissopoulos et al., 2003).

Raw sewage from the conventional wastewater treatment plant of Thessaloniki, after screening, was oumped into a covered deep anaerobic pond at a rate of Q = 120 m?/d. Approximately 1/4 of the lischarge (Q = 30 m?/d) was feeding Line | and the rest 3/4 (Q = 90 m?/d) Line 2. Line 1, after the anaerobic pond consisted of a facultative pond and two maturation ponds. Line 2 layout was similar 0 line A, but with a recirculation of 180 m?/d from the last maturation pond to the facultative pond. Line 3 received primary treated effluent (Q = 50 m*/d) from the nearby treatment plant and Sonsisted again of a facultative pond and two maturation ponds (Figure 1). Treated effluent from the hree lines was stored into a reservoir and then used for irrigation of edible and non-edible crops, ifter algae being reduced by an intermittent slow sand filter of 100 m? (Parissopoulos et al., 2003).

Tat HWIGIVIGUadl SYSUCHIS .s already mentioned, WSP are not so popular in Greece. An exception to this is two Prefectures 11 1e North (Kavala and Serres), where 13 systems have been constructed since 1982 servin; ommunities in the range 600-3,000 p.e. All these systems consist of a primary facultative pon sllowed by one to three maturation ponds. Most of the ponds have operation and maintenanc: roblems (odours, rooted plants etc.). The effluent of most of the WSP in operation are used b' ycal farmers for crop irrigation; this is a sensible practice in principle but needs to be done unde nproved information and control (Tsagarakis et al., 2000). All these systems are presented it able 2. As can be seen from this table, a number of systems were initially designed but for politica -asons were not finally implemented. This is a consequence of the notion that an appropriat vastewater system facility should involve installation of complex systems and machines.

Table 2. Existing WSP systems in Greece 'As reported at the designs or estimations where unavailable °A: Anaerobic, F: Facultative, M: Maturation, RF: Rock Filter

Table 3. Designed, final constructed, and the proposed dimensions of the WSP in Kokkinochoma. Grab samples where taken to evaluate the performance of this WSP system. Looking into these results should be taken into account that the effluent was from the first maturation pond as the second and last one was empty due to excessive pumping from farmers (Table 4). Also the actual average daily flow was 50 m? while the designed was 135 m*. Therefore extended retention times had resulted in better performance of the system.

Table 4. Qualitative data for the WSP of Kokkinochoma.

Aerated ponds have been used to treat tomato processing wastewater for a major tomato industry factory (SEVATH) at the Northern part of Greece. This system consists of 3 ponds in series and receives 8,000 m?/d wastewater (1520 kg BOD/d) only for 2 months of the year when there is major tomato production. All ponds have a 5 meter depth. The volume of the first pond is 20,000 m3, while the second and the third is 16,000 m? each. The first one is a hybrid activated sludge system, the second one is partially aerated, while the third one operates both as a maturation pond and a reservoir. A minimum of 2 mg O,/L concentration is maintained in the first pond. A layout of the nilantic o1ven in Fionre 2

The organic loading from the study was estimated on the basis of 135 m?/d (Q) x 200 mg/L (BOD; concentration), which equals 27 Kg BODs/d. The real loading today is 50 x 402.9 = 20 Kg/d. This mistake was perpetrated as a result of reliance on a BODs production of 30 g/p.e.-d (for 900 p.e.). Similarly, SS is estimated 135 m?/d(Q)x250 mg/L(SS concentration)= 33.75 Kg/d, while real today quantity is 50m?/d(Q)x239.lmg/L(SS concentration)= 12 Kg/d. Calculating the sample results, it can be concluded that the serving p.e. of the system is about 330 p.e., with an average wastewater production 151 L/p.e.-d. The pond layout of the system is given in Figure 2.

Table 5. Qualitative and quantitative data of the plant Design problems

in some cases soil permeability was not examined and systems were designed in office, without any -onsideration of the site. This resulted in some of the ponds operating like absorption pits. As an llustrative example of this problem is the system of Mavrolofos, where the first pond was never filled.