The quantification of the nitrogen and phosphorus reduction potential from households not connected to public sewerage should be based on national average specific loss figures of nitrogen and phosphorus into water bodies, taking into account the population size, the equipment used, treatment methods, pathways of discharge and the distance from the water bodies. Because not all Contracting Parties were able to provide the information needed to calculate the reduction potential, we decided to use a very simplified approach (OSPAR Guidelines for Harmonized Quantification and Reporting Procedures for Nutrients, HARP-NUT, 2004).
Calculation principle: It was assumed that a large part of the unconnected households have improper treatment equipment, corresponding to simple separation tanks. For such households, the potential improvement can be calculated as the difference between specific load from the households with water flushed toilets and with no specific external treatment (except sedimentation tanks) and those fulfilling the HELCOM Recommendation 28E/6 treatment level (0.24 kg phosphorus per year) as compared to 0.43 kg phosphorus per year (OSPAR Guideline 5). No estimate was made for nitrogen because the maximum permissible daily per capita load for total nitrogen according to the HELCOM Recommendation is higher than the indicative specific loads of nitrogen in the respective OSPAR guidelines.
Three different scenarios (A to C) are presented in the figure below according to the share of total scattered population living in the zone where reduction measures can be assumed to benefit water courses. The results are given both at source and as estimated amounts ultimately discharged into the Baltic Sea. The retention of nitrogen and phosphorus in the catchment area was assumed to be 30% in the whole catchment area. No estimate of the reduction potential for rain and storm water overflows were performed because as statistics and data are poor, but it was assumed that the reduction potential is lower than that for scattered dwellings.
Phosphorus net reduction potential (in t a−1) from scattered dwellings (about 21 million people) according to three scenarios. A: The whole population in areas where reduction can be assumed to benefi t water courses; B: 2/3 of the population; and C: 1/3 of the population