Land disposal of treated sewage effluent

Abstract

Land disposal of treated sewage effluent is a relatively minor form of sewage treatment in New Zealand. The reluctance to use this effective method of tertiary treatment stems from a lack of knowledge of the potential health hazards which may result from effluent application onto land. Consequently, a greater understanding of the operating characteristics of this technique is required before its use becomes more widespread. This project focuses on the effects on groundwater quality of nitrate-nitrogen contained in the drainage water.

The project's aim was to develop a management system which would reduce nutrient losses to the groundwater. To accomplish this aim, nutrient losses were quantified by collecting percolating soil-water samples and dry matter yields for two management systems; fat lamb and hay production. Overseas experience indicated that the harvesting of a crop irrigated with sewage effluent minimised nutrient losses. One crop, reed canarygrass, was most efficient in removing nutrients contained in effluent.

Reed canarygrass' performance under effluent irrigation conditions was investigated and compared with ryegrass/clover pasture. Additional investigations into the potential of reed canarygrass as a stock feed and as a crop for hay production were considered in light of economic and energy factors. As this grass is relatively unknown in New Zealand, the investigation will provide useful guidelines on its ability to perform under New Zealand conditions.

Drainage losses of nitrate-nitrogen were shown to be minimised by harvesting reed canarygrass. Decisions on the future use of the grass will be governed by the value placed on high quality groundwater, because in economic and energetic terms, the harvesting of a crop was seen to be unfavourable.