Design and performance of constructed wetlands for the treatment of parlour washing effluents

Abstract

Initially designed for the treatment of domestic wastewater, Vertical Flow Reed Bed Filters (VFRBF) have been adapted for washing parlour effluents to study the feasibility of the treatment of such effluents instead of their storage and land spreading. This work has been done within the framework of the Programme of Control of Pollution of Agricultural Origin (PMPOA). It was necessary to adapt the design of VFRBF to minimise the investment costs. Several experimental sites have been built, monitored and progressively adapted. Following a septic tank of 3 to 9 m3, the best design was considered to be: 2 stages of VFRBF in series each composed of 2 filters which are alternately fed on a weekly rhythm. The total active area is approximately equivalent to 0.25 m2 per dairy cow for the washing effluent of the milking machine and milk-storage tank. It is increased to 0.4 m2 per dairy cow when the washing effluents from the floor and walls of the milking parlour are added to the previous ones. The 1st stage filters takes up 65 % of the total area. Based on a survey done in 3 farms, the concentrations achieved a range of 50 to 180 mgL-1 in COD, 5 to 75 mgL-1 in BOD5, 10 to 60 mgL-1 in SS. The nitrification, which can be considered as a good indicator of the redox potential within the filter media, was not complete but much more effective than in buried intermittent sand filters [ISF] studied for the same purpose but some of them have clogged prematurely, which did not allow to approve this system.