Phosphorus removal in on-site wastewater treatment plants is normally obtained by chemical precipitation. Aluminium-based chemicals are the favoured coagulants as they are not affected by redox potential. On-site wastewater treatment package plants do not have separate sludge treatment facilities, and sludge is normally collected on an annual basis. This can potentially increase the risk of phosphorus release into the water phase, subsequently reducing treatment efficiency. This study aimed to detect release of phosphorus as a result of chemical and biological processes. Variables in the study were time, aluminium dosage and pH. Wastewater sludge was monitored for 46 weeks to investigate the different mechanisms of phosphorus release and the longevity of the aluminium treatment involving varying aluminium dosages. Phosphorus compounds were analysed based on a modified Psenner sequential fractionation method. Both pH and aluminium dosage affect the longevity of the phosphorus retention of chemically precipitated wastewater sludge, where sufficient longevity is obtained with pH control and increased aluminium dosages. Chemical dosages similar to what is considered normal levels are sufficient to retain the phosphorus in the sludge for annual sludge collection intervals. Release of soluble phosphorus was attributed to microbial activity and crystallization of Al-hydroxide complexes.