Qualitative monitoring of a treated wastewater reuse extensive distribution system: COD, TSS, EC and pH

During a five-month summer period, samples of tertiary treated wastewater flowing in an extensive distribution system composed of storage tanks and pipes, were collected at two-week intervals from 21 different sampling points, including the exit from the wastewater treatment plant (WWTP). The WWTP producing this effluent treated wastewater from one of the most popular European tourist resorts on the north coast of the island of Crete, at the southernmost point of Greece. Crete is a semiarid region where 80 % of the freshwater resources are consumed by agriculture. More than 3 000 000 tourists visit the island during the summer period and the average summer equivalent population treated by the plant exceeds50 000, falling to under 5 000 in the winter. The samples were analysed for chemical oxygen demand (COD), total suspended solids (TSS), electrical conductivity (EC) and pH. The average COD and TSS in the WWTP exit were within the reuse limits for orchard irrigation, being 80 mg/l and 25 mg/l respectively. Both recorded higher values in the other sampling points, but still as an average below the above-mentioned limits. COD values along the distribution system presented a strong correlation with the WWTP’s effluent quality, affected mainly by the condition of the collector, whereas TSS presented a completely different behaviour. EC and pH exceeded the optimum operation and reuse guidelines, mainly due to excessive septage in the WWTP. However, both presented a stable and predictable behaviour in correlation to the effluent quality, in terms of both distance and time, similar to that of COD. In terms of these parameters it is safe to suggest that, , achieving the required quality standards in the WWTP exit , the wastewater quality should be considered adequate for reuse for irrigation.