Functional requirements for small sewerage systems - Basis for decision on requirements for reduction and discharge levels of faecal microorganisms from small sewerage systems

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Background
The National Board of Health and Welfare, together with the Geological Survey of Sweden (SGU), carried out a national monitoring project in 2007 on drinking water from private water sources. The summary of the results of the water analyses shows that the drinking water quality in many individual water sources was poor. Only about 20% of all samples were safe and the same proportion was unsafe. The worst drinking water was found in dug wells, where almost 35% of samples were unfit. Microbiological contamination was the most common cause of unfit drinking water (National Board of Health and Welfare 2008). One reason for the large number of contaminated wells may be poor design of the drinking water well resulting in surface water seepage or groundwater affected by surface water, but in many cases contaminated groundwater is due to poorly constructed or missing nearby sewage facilities. Often there are only sludge separators without subsequent treatment steps (Swedish EPA 2012). Microbial impact in the form of fecal contamination from sewage and manure means that pathogens such as viruses, bacteria and parasites may be present in the water, posing a risk to human health (Geological Survey of Sweden 2013). Small drinking water systems (< 50 PE) have also been found to be associated with problems internationally. European studies have found that these facilities are often fecally contaminated (Yip-Richardson et al. 2009) with real risks of waterborne outbreaks (Said et al. 2003). Furthermore, in the EU Healthy Water project, Risebro et al. (2012) showed that children under the age of 10 living in areas with private water, or with drinking water from a small facility with fecal contamination, had a significantly higher risk of contracting gastroenteritis than the general population, in line with what has been reported from developing countries. Although the situation in Sweden is probably not so bad, there is an equity aspect to consider, i.e. that everyone living in Sweden should have the right to healthy water, regardless of where they live.
Misplaced, incorrectly dimensioned or inadequate small sewage plants probably constitute a large part of the problem described above and there is a need to “upgrade” most sewage solutions to something that can be healthily sustainable. In 2006, the Swedish EPA issued new general advice on small sewers, divided into normal and high protection levels (Swedish EPA 2006). While there is guidance in the form of reduction requirements for nitrogen, phosphorus and oxygen-consuming substances, the health-related requirements are treated more arbitrarily, which gives the environmental and health protection offices little support in their enforcement work. From a legal certainty point of view, problems are also created as the interpretation of “Discharge of wastewater does not contribute to a significantly increased risk of infection or other nuisance” can be anything from small sewers being ignored to discharging water that is of good bathing water quality. Setting targets for the reduction and discharge of microorganisms will thus not only provide a general improvement in the quality of the environment from a hygienic point of view, but at the same time these targets will provide support in the enforcement work, which can also be harmonized between municipalities and regions in the country.

Risks of transmission
Looking at the risks of different types of diseases that can be spread through small wastewater facilities, these are generally common stomach diseases such as winter vomiting, campylobacteriosis and salmonellosis. As a result of the differences presented in Table 1 above, wastewater from a small plant may be free of pathogens for much of the year. However, in the event of infection or clinical illness of someone connected to the plant, the dilution of these pathogens is not as great as in a municipal treatment plant, and peaks of high levels in the wastewater may occur. In order to even out these peaks, more barriers are therefore needed, not only to separate pathogens, but also to even out and dilute the contaminants in both time and space.
Barriers against the spread of infection
A sewage treatment plant must protect against the spread of infection and the spread of antibiotic-resistant bacteria to animals and humans. This is done by creating protective measures (barriers) in order:
(1) separation of fecal material at source (if possible): by separating the fecal matter from the water entering the plant, a robust reduction of pathogens1 in the wastewater (now called BDT water) is achieved.
(2) treatment to equalize concentrations/flows and reduce pathogens in a facility closed to the public, which can be e.g. a septic tank, a mini-treatment plant and/or a soil bed/enhanced infiltration. It is for this barrier, the facility, that reduction requirements will be specified (see Annex 1).
(3) retention, post-treatment in a land/water area shielded from the public before discharge to the receiving water. For exposure via groundwater, retention consists largely of what is known as protective distances. The ability to use the retention in soil, i.e. the ability of the soil to receive infiltrated wastewater, is of great importance for the classification of the protection level with regard to health (see below).
(4) exposure, location of discharge point so as to minimize exposure to humans and animals. In addition to reduction requirements for treatment, emission requirements for exposure are proposed. Depending on the functioning of the installation and the retention, the effluent at the point of discharge shall be of satisfactory bathing water quality (see Annex 1).

Reduction requirements: This is the core of the task, how can an appropriate level be determined? The approach has been to use a quantitative microbiological risk assessment for the organism (norovirus) most likely to spread via small drinking water sources (E. Hallin, pers. comm.) and to ensure that this spread is at a level in accordance with the WHO health guidelines. With good potential for reduction in soil and basic protection, 3 log reduction (99.9%) was considered an appropriate level. In conditions where retention cannot be relied upon, i.e. enhanced protection, the facility needs to be able to remove up to 5 log (99.999%) of pathogens before the water can be infiltrated with the potential to impact a groundwater source. The Swedish National Board of Housing, Building and Planning’s description of tests for CE marking is a proposal for how this reduction can be validated (Appendix 1). Alternatively, one can start from default values for incoming water (e.g. 107 CFU E. coli/100 mL) and only measure outgoing water. The guideline value for E. coli out of the plant at the normal level would then correspond to 104 CFU/100 mL, for high protection level 100 CFU/100 mL. Sampling may be simple in theory but not so easy to implement in practice, which is why the question of how best to validate the functioning of plants should be investigated separately (see further control below).

3. Discharge requirements: Despite three log reduction over a facility, there may be a risk of contamination in case of direct exposure (Table 6), which is why it is proposed to post-purge or complicate exposure of the treated water (see further retention and exposure below). The proposal is that the water when it comes into “daylight” outside the property boundary should be of a satisfactory bathing water quality (Table 3) and that samples should not exceed 1000 CFU E. coli/100 mL and intestinal enterococci < 400 CFU/100 mL (Appendix 1), which corresponds to about 4 log reduction. This means that in the normal level of protection, a backwash may be needed in an area screened from the public or that