Explaining the mysteries of nitrogen and phosphorus in on-site wastewater

Nitrogen and phosphorus are the two most commonly examined nutrients in on-site wastewater treatment, effluent disposal and water recycling. Nitrogen and phosphorus concentrations are determined due to their perceived roles as potential pollutants with them often described as causing both environmental and human health problems. Exactly what harm is caused directly by nitrogen and phosphorus is a matter of debate. While these nutrients are not specifically regulated in AS/NZS 1547:2000, most Australian States and Territories require on-site treatment systems to achieve set reductions in nitrogen and phosphorus before a technology gains type approval. On-site systems over 20 Equivalent Persons (EP) require individual licences in most Australian jurisdictions. These licences generally set parameters for nitrogen and phosphorus, although no standard legislative framework exists either within or between relevant jurisdictions to ensure that these licence requirements are consistent. Water recycling management plans and/or licences also require, in most cases, nitrogen and phosphorus concentrations of the treated effluent to be established and to be monitored in the application area. This paper will detail the most common sources of nitrogen and phosphorus in raw wastewater, and how the specific nutrient cycles operate within typical on-site treatment systems. The changes that occur to these nutrient cycles once the treated effluent is applied to soil will be examined and the associated risks defined.