PROJECT DESCRIPTION
BACKGROUND
The use of Waste Stabilisation Ponds (WSPs) to treat wastewater has increased greatly since the end of the seventies and is frequently chosen by small communities. There are now some 2,500 and 3,000 WSP systems in France which meet the limits for effluent quality set by national regulations (i.e. French quality level D3). However, level D4 (<125 mg/L COD; <25 mg/L BOD5) is rarely achieved due to the presence of algae. In effect, with the classical WSP, these limits cannot be achieved and a complementary treatment is required. Several technologies have been proposed for upgrading pond quality effluent. A further consideration is management of the small amount of sludge (about 2cm/year of wet anaerobic sludge) produced to keep the pond performing properly. Desludging represents a major factor of maintenance operational costs in WSPs.
OBJECTIVES
In this context, the main aim of the LILIPUB project was to build a demonstration unit, installed at Aurignac (Haute-Garonne) with a view to processing domestic wastewater of 300 inhabitant equivalents. The new treatment line was to use an association of WSP and Intermittent Sand Filters (ISFs), plus a sludge line. The objectives were as follows: - To associate pond and ISF to achieve the French D4 quality level and complete nitrification. - To treat storm-waters drained by combined sewers. - To allow an easier sludge treatment and management. The final aim of the project was to propose a wastewater treatment system for rural districts (with combined sewerage network), which was easy to operate.
RESULTS
The system developed by the LILIPUB project has allowed for a notable improvement in the quality of the effluent (reaching D4 level) and is a realistic technical solution for treating wastewater in rural communities. The sewage treatment system is of a combined type and includes a pond equipped with a settling tank, infiltration/percolation plants, and sludge drying reed beds to treat the sludge which is pumped daily from the settling tank. The system operated with a hydraulic load reaching 80 cm per day. The results from the demonstration unit, which was successfully set up at Aurignac, provide major elements for designing new wastewater treatment plants (WWTPS) and enhancing pond systems. In terms of sizing new WWTPs, two configurations were proposed: (1) 2 ponds and filters or (2) 1 settling tank, 1 pond and filters. The innovation of the demonstration project lies in the association of the classical pond system with the infiltration in sand filters (planted or not) and with the sludge treatment (extraction and sludge dewatering reed bed), as a wastewater treatment system for rural communities. The process is now being duplicated in another French community. The environmental benefits obtained are the following: - The total surface needed is smaller in comparison to classical ponds systems and thus represents less of a constraint for small communities. - Improvement of water quality discharge into the receiving bodies with low level of oxydable compounds. - A high capacity of storm water treatment (suspended solids are drained during such events). In classical plant configurations, the pollutant mass, which is in some cases non-negligible, is usually discharged directly into the receiving bodies. - An ecological in situ treatment of sludge which allows a diminution of the volume of sludge produced. Furthermore, the sludge is mineralised and aerobic and its withdrawal is easier and can be done without stopping the water treatment line. No economic study for this treatment line was carried out, however the operating costs were estimated to be around €3218 per year (which excludes experimental costs and treatment of sludge).The system developed by the LILIPUB project has allowed for a notable improvement in the quality of the effluent (reaching D4 level) and is a realistic technical solution for treating wastewater in rural communities. The sewage treatment system is of a combined type and includes a pond equipped with a settling tank, infiltration/percolation plants, and sludge drying reed beds to treat the sludge which is pumped daily from the settling tank. The system operated with a hydraulic load reaching 80 cm per day. The results from the demonstration unit, which was successfully set up at Aurignac, provide major elements for designing new wastewater treatment plants (WWTPS) and enhancing pond systems. In terms of sizing new WWTPs, two configurations were proposed: (1) 2 ponds and filters or (2) 1 settling tank, 1 pond and filters. The innovation of the demonstration project lies in the association of the classical pond system with the infiltration in sand filters (planted or not) and with the sludge treatment (extraction and sludge dewatering reed bed), as a wastewater treatment system for rural communities. The process is now being duplicated in another French community. The environmental benefits obtained are the following: - The total surface needed is smaller in comparison to classical ponds systems and thus represents less of a constraint for small communities. - Improvement of water quality discharge into the receiving bodies with low level of oxydable compounds. - A high capacity of storm water treatment (suspended solids are drained during such events). In classical plant configurations, the pollutant mass, which is in some cases non-negligible, is usually discharged directly into the receiving bodies. - An ecological in situ treatment of sludge which allows a diminution of the volume of sludge produced. Furthermore, the sludge is mineralised and aerobic and its withdrawal is easier and can be done without stopping the water treatment line. No economic study for this treatment line was carried out, however the operating costs were estimated to be around €3218 per year (which excludes experimental costs and treatment of sludge).
