Περίληψη
Στην Ελλάδα έχουν ήδη κατασκευαστεί και λειτουργούν φυσικά συστήματα (δεξαμενές σταθεροποίησης και τεχνητοί υγρότοποι) για την επεξεργασία αστικών υγρών αποβλήτων σε μικρούς οικισμούς, που διαθέτουν αποχετευτικό δίκτυο. Πολλοί όμως μικροί οικισμοί στην Ελλάδα εξυπηρετούνται ακόμη από βόθρους, π.χ. d the ponds and the system operated under duckweed covering (duckweed period). Over the following 5 months a gradual decay of duckweed began, and reeds (Typha) started to develop in the second pond (transitional period). Reeds were removed by artificial means at the end of this period. For the last 13 months of the experiment, the pond operation was based exclusively on algae (algal period). The average inflow rate was 44 m³/day, which corresponds to a small Greek village of approximately 300 p.e. The seasonal water supply of the system was varying with the higher value in the summer. This was done to simulate the population increase in many Greek settlements during the summer months because of tourism. Grab samples were collected from the vacuum truck and the exit of each pond on a weekly basis and nineteen (19) parameters were examined in the laboratory. On-site measurements of dissolved oxygen (DO) and water temperature (Tw) were made biweekly. The dense duckweed mat at the beginning of the experiment suppressed algae growth, limiting this way the DO production and prevented pH from elevating. Despite the anoxic pond conditions (DO < 1 mg L⁻₁), however, noxious odours were not observed, even during this period. Results showed that the values of 15 parameters were reduced at the exit of the last pond and those of other 4 parameters were increased. The pond system reduced the outflow of E. coli (99.71%), BOD₅ (94%), enterococci (92.74%), COD (61%), TSS (76%), NH₄⁺-N (75%), VSS (72%), TKN (71%), TN (70%), [N-(NO₃⁻+ NO₂⁻)] (52%), chl-a (52%), VS (36%), VDS (12%), ο-PO₄³⁻-P (16%) and alkalinity (4%). On the contrary, increased values were observed for TDS (74%), EC (50%), TS (41%) and pH (8%) after treatment in the stabilisation ponds, as was expected. Seasonal differences in the reduction of organic (BOD₅, TSS), nutrient (NH₄⁺-N, ο-PO₄³⁻-P) and microbial (E. coli, enterococci) load, between algal and duckweed period, were mostly observed in the warm season. Regarding the organic load, as expressed by BOD₅ and TSS, the treated septage complied with the 91/271/EEC Directive either as reduction or as concentration. COD concentration was marginally higher than the required one, except for the warm algal season. The percent reduction of TN was within limits, while the phosphorus removal was satisfactorily achieved in the warm algal period. Finally, E. coli concentration complied with the WHO limit of 1000 cfu/100 mL. Sludge levels were increasing and lowered according to season. Heavy metals were not found in remarkable concentrations due to the wastewater origin. An important point of the research was that a pond system can operate satisfactorily without the need of duckweed removal which is a difficult task for the local administrators of these systems. Empirical models, reflecting the particular local conditions, were developed for each pond. Percent reduction and concentration of BOD₅, TN, NH₄⁺-N, ο-PO₄³⁻-P, E. coli and enteroccocci were expressed as a function of various physico-chemical parameters. Initial concentration and pH were found to explain the most of the variances. Additionally, simple design models based on BOD₅, TN and E. coli removal were developed for predicting the hydraulic retention time and, therefore, the pond dimensions. These models were based on the estimation of the decay rate (Kb, d⁻¹) for each parameter with the aim to apply it on a larger scale and under different experimental conditions. The results were verified with other data from 36 regions in 21 countries, throughout the five continents.
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