In Ireland, the most common method of disposal of dairy parlour washings is by land spreading. This treatment method has numerous problems, namely high-labour requirements and the potential for eutrophication of surface and ground waters. Constructed wetlands are commonly used for treatment of secondary municipal wastewaters and they have been gaining popularity for treatment of agricultural wastewaters in Ireland. Intermittent sand filtration may offer an alternative to traditional treatment methods. As well as providing comparable treatment performance, they also have a smaller footprint, due to the substantially higher organic loading rates that may be applied to their surfaces. This paper discusses the performance and design criteria of constructed wetlands for the treatment of domestic and agricultural wastewater, and sand filters for the treatment of domestic wastewater. It also proposes sand filtration as an alternative treatment mechanism for agricultural wastewater and suggests design guidelines.

Accumulation of biomass and deposition of suspended solids at the surface of a sand filter can lead to clogging of the filter media. A laboratory intermittent sand filter column, which included three sand strata, was operated for a period of 806 d before failure occurred through surface clogging. Upon dismantling the column, the cause and effects of the surface clogging were investigated. The main mechanism responsible for sand clogging appeared to be biomass buildup. Maximum loss on ignition of filter media samples was 2.35%, and it occurred in the upper 0.01 m of the sand. There was a reduction in field‐saturated hydraulic conductivity in the top 0.01 m of the upper sand stratum from a value of 1.9 × 10−3 ± 1.7 × 10−4 m s−1 (for virgin sand with an effective size, d10, of 0.45 mm) to 3.5 × 10−5 ± 7.5 × 10−6 m s−1 The soil‐water characteristic curve, which relates the volumetric water content (θv) to the soil suction, also reflected the changes in the filter media due to clogging. The water‐holding capacity greatly increased as biomass accumulated in the filter media. Scanning electron microscopy (SEM) confirmed the existence of a clogging organic layer on the surface of the top sand layer.

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A stratified sand filter column, operated in recirculation mode and treating synthetic effluent resembling high-strength dairy wastewaters was studied over a 342-d duration. The aim of this paper was to examine the organic, total suspended solids (TSS) and nutrient removal rates of the sand filter, operated in recirculation mode, under incrementally increasing hydraulic and organic loading rates and to propose a field filter-sizing criterion. Best performance was obtained at a system hydraulic loading rate of 10 L m(-2) d(-1); a higher system hydraulic loading rate (of 13.4 L m(-2) d(-1)) caused surface ponding. The system hydraulic loading rate of 10 L m(-2) d(-1) gave a filter chemical oxygen demand (COD), TSS, and total kjeldahl nitrogen (TKN) loading rate of 14, 3.7, and 2.1 g m(-2) d(-1), respectively, and produced consistent COD and TSS removals of greater than 99%, and an effluent NO(3)-N concentration of 42 mg L(-1) (accounting for an 86% reduction in total nitrogen (Tot-N)). As the proportional surface area requirement for the sand filter described in this study is less than the recommended surface area requirement of a free-water surface (FWS) wetland treating an effluent of similar quality, it could provide an economic and sustainable alternative to conventional wetland treatment.