On-site domestic wastewater treatment by MHEA® technology:: Case study in Belgium

In rural and low population density areas, on-site household wastewater treatment might be recommended due to economical or technical reasons (cost of sewers, topography constraints, etc.). In such areas, the demand for alternative technologies, such as extensive wastewater treatment systems or constructed wetlands to treat domestic wastewaters, is increasing in particular due to the evolution of current legislation and to the population's concerns about environmental issues. The MHEA (R) research team in Belgium (Station Experimentale de Viville) introduced a technical proposal to adapt the MHEA (R) technology to the constraints and to the legal requirements of the Walloon Region (Belgium) related to on-site treatment for suspended solids (60 mg L-1), COD (180 mg L-1) and BOD 5 (70 mg L-1). A pilot MHEA (R) wastewater treatment system has been sized, built and integrated into a garden in order to treat the wastewater produced by a household (5 PE). It has been in operation since May 2002. The MHEA (R) system covers a net treatment area of 32.5 m(2) (6.5 m(2) PE-1) and consists of a succession of 3 ecosystems (aquatic, semi-aquatic planted with Typha, terrestrial planted with Salix and Alnus). The MHEA (R) system is supplied by the water outflowing from a conventional septic tank. The MHEA (R) system has been monitored twice a month since May 2002. Water samples from each component of the system have been analysed according to standard methods. The paper presents the results of the monitoring (36 samplings runs from May 2002 to November 2003) and the performances of the MHEA (R) system for primary treatment (SS), secondary treatment (COD, BOD5), tertiary treatment (N, P) and for disinfection. For SS, COD and BOD5, the MHEA (R) system provides excellent and stable performances during both dormant and growth periods with mean removal efficiencies of 93% for SS, 94% for COD and 97% for BOD, and mean outflow concentrations of SS, COD and BOD 5 of 7 mg L-1, 40 mg L-1 and 15 mg L-1, respectively With regard to N and P, wastewater inflow concentrations are very high (TN: 148 mg L-1 and TP: 17.6 mg U) and the removal efficiencies show differences between dormant and growth periods. In the 2003 vegetation period, the mean removal efficiencies reached 77% for both parameters with outflow concentrations of 37 mg IN L-1 and 4.1 mg TP L-1 whereas during the dormant season, N removal efficiency reached only 54% and P removal efficiency remained round about 70%. Otherwise, during the first 6 months of operation, the outstanding P retention is dependent on the specific physico-chimical characteristics of the soil. As regards disinfection, the system gives up to 3 - 6 log(10) units reduction for both faecal coliforms and faecal streptococci. Disinfection capacity was lower during the winter (long period of permanent frost).