Renewal of a private household soil filtration system in Nurmijärvi
Solution to which problem
Nurmijärvi is a suburban area north of Helsinki in southern Finland, with a lot of detached houses in the area, located far away from the centralized sewer system. Old soil filtration system installed for the private household was not functioning anymore, which deemed the necessity for a new system. As the new household wastewater regulation requires sufficient purification efficiency, thousands of small wastewater treatment systems in Finland need to be renewed, Nurmijärvi pilot is used as a good example on how the renewal can be done.
The implementation was done for one household, consisting of four inhabitants. The house does not have a possibility to join a larger municipal wastewater treatment network. The total amount of inflow was assumed as 575 L per day, treating the water for 4 people (144 L per day per person). The influent BOD load was assumed as 200 g BOD, 56 g nitrogen and 8.8 g of phosphorus per day. The old wastewater treatment solution was used with small modifications to achieve better BOD and nutrients reduction.
The old solution for the private household in Nurmijärvi consisted of four septic tanks, a small pump (0.4 kW) and Uponor ground filtration pipe packet. The filtration material was sand and in addition crushed stone was used. The system was planned to be usable for 15 years, while by the time of renewal it had been working for 22 years. As the soil filtration was clogged, action was needed with the system.
For the new installation, the same septic tanks were used, while the pipes were changed, and another pump was added for water pumping from septic tanks. The four septic tanks were 600-1200 mm by diameter and around 2200 mm high. The principal of the system is to lift the wastewater from the septic tanks through a pump higher so that after the filtering field the waters can be discharged to the nearby ditch. Sand, crushed stone and filtration textile were bought besides the new pipes and pump for the reconstruction and used for the soil filtration system. The new sand filtration layer removes phosphorus more efficiently than the old one. As another pump was added to the system, the total energy consumption of the system rose from 18.6 kWh per year to 37 kWh per year.
The new soil filtration system was set up in the summer 2017. Every side of the absorption field (1-4 point, 25 cm and 100 cm depths) soil results have shown that, from October 2017 until September 2018 total and soluble phosphorus, soluble nitrogen, ammonia and nitrates nitrogen concentrations in all point were lower compared to the concentrations in 2017. But from October 2017 until September 2018, NO3-N concentration in all points was higher compared to the values in 2017. From October 2017 until September 2018, Coliformic bacteria, Escerichia coli, fecal colifomic bacteria and fecal streptococci values were the same compared to the values in 2017.