Manurial value of septic-tank sludge on grassland

About 5% of the UK population is served by septic tanks and cesspits. In some districts this is applied to farmland. The Water Research Centre and South-West Water collaborated in establishing a field trial to provide knowledge of the manurial value of septic-tank sludge. The trial was designed to investigate the value of nitrogen (N), phosphorus (P) and potassium (K) contained in the sludge and applied over a 3-year period to grassland. The approximate proportions of total nutrients present in the sludge applied which became available to ryegrass as inorganic fertilizer nutrients were:
Barley grown in the fourth year when no sludge was applied showed a significant response to residual sludge N and P remaining in the soil. It is concluded that septic-tank sludge can supply useful amounts of N and P, and, unlike other forms of sewage sludge, significant quantities of K to grassland. Estimates of the nutrient value should be based on analysis for dry solids, and total N, P and K. Septic-tank sludge is usually low in metal content and no metal contamination problems should arise when it is applied to land.