Bättre enskilda avlopp i Sigtuna kommun
möjligheter för bebyggelse i Odensala socken
There are around 855 000 on-site sewage systems in Sweden and some 1 800 of these are located in the municipality of Sigtuna. The Sigtuna local authority has set the goal that all sewage systems with insufficient function should be improved before the end of year 2010.
A malfunctioning on-site sewage system may cause three main problems: spreading of diseases, discharge of eutrophicating compounds and wastage of resources. The municipality strives to reduce these problems already at the stage of granting permits for installation of on site sanitation systems. The municipality has recently adopted new guidelines, which demands certain minimum reductions for different compounds. The guidelines also state that it is preferred that neighboring house-owners cooperate in jointly built and operated sewage systems and that the system should be able to recycle plant nutrients. Systems with infiltration to groundwater are accepted even though their function is hard to monitor.
This report focus on a specific area within the municipality but the results and discussion are also relevant to other similar areas. There are 54 households in the selected area. These were sorted into four categories according to their circumstances. Around 40 % of the households in the area have a legal sewage system and infiltration to groundwater and filter beds are the most common systems.
In this report package treatment systems and filter beds ? combined with either chemical
precipitation or urine separation are evaluated, as well as connection to a large sewage treatment plant. These systems are compared regarding expected reduction, recycling potential and economy.
The systems studied ought to meet the reduction requirements set by the local authority, i.e. reduction of at least 50 % of the nitrogen and 90 % of both the phosphorus and oxygen demanding compounds. However, the reduction reported in different studies varies greatly between them and also between installations in the same study. A system with filter bed combined with chemical precipitation might have some difficulties to achieve the target concerning nitrogen removal while on the other hand urine separation might have problems to meet the target for phosphorus reduction.
To connect to the municipality?s sewage treatment works means that theoretically 19 % of the nitrogen and 86 % of the phosphorus can be recycled and both package treatment systems and chemical precipitation in combination with filter bed have similar recycling potentials. Systems with urine separation differ by greater potential to recycle nitrogen but smaller to recycle phosphorus. Achieving recycling of plant nutrients implies a functioning organization including transport, quality control and interested users of the recycled nutrients.
The yearly cost for a single-family treatment system is around 10 000 SEK. The cost is lower when two or more households share a joint system. Connecting to the central treatment plant appears to be the most expensive alternative but also a package treatment system for one household may reach a yearly cost around 13 000 SEK.
Suggestions for a better sewage treatment in the area include upgrading of the existing systems and new systems. This report mainly focuses on the possibility to implement shared treatment facilities and such installations can be applied by at least five groups of households in the area.
The discussion section of this report gives the local authority advice on how to act when in contact with the house-owners and how to work to achieve better sewage treatment.