Nuisance provisions of the Public Health etc (Scotland) Act 2008: guidance

Procedural guidance on the statutory nuisance provisions outlined in the Public Health etc. (Scotland) Act 2008.




1. The major nuisance species derived from WWTW filter beds are the moth fly, Tinearia (= Psychoda) alternata (filter/drain fly), and the window gnat, Sylvicola (= Anisopus) fenestralis (Scopoli).17 other Psychoda species, such as Psychoda albipennis Zetterstedt (= Psychoda severini Tonnoir) and Psychoda cinerea Banks, may also occasionally emerge in high numbers.

2. Whilst none of the filter-derived insects sting or suck blood, they cause annoyance and can be nuisance in domestic situations. Similarly, whilst there is no evidence that they transmit disease, in the case of Tinearia / Psychoda species, enteric bacteria have been found on their bodies and they may cause allergic reactions through the shedding of hairs and scales and through post-mortem disintegration. More seriously, there have been reports of human urogenital myiasis caused by psychodids although there are no reports from the UK, probably because infected humans are required to have an intimate association with polluted water for infection to occur.

3. The biology, ecology and thermal requirements of the Tinearia / Psychoda species and S. fenestralis allows for predictions on the likelihood of high population outbreaks to be made. The chironomids Lymnophyes minimus (Meigen) and Metriocnemus sp . may also occur in numbers sufficiently high to cause annoyance to the general public on occasion. Dipterans found in association with filter beds include sphaerocerid, ephydrid, ceratopogonid and sepsid flies.

4. Mosquitoes (Culicidae) have been reported as being associated with WWTWs but their larval development is not typically associated with the filter bed. Where problems occur, these can be related to other niches created in and around the WWTWs where areas of standing stagnant water are present.

5. The interactions within and between macro invertebrate species in WWTW filter beds indicates that species competition plays an important role in populations of any given insect species, and that the likelihood of the numbers of any given insect species rising to nuisance status may be governed by what other species co-occur within a given filter bed. Therefore, species richness can be seen as an important factor in regulating fly populations and also in maintaining the efficiency of the filter through preventing the excess accumulation of film.

6. Low, but variable, thermal thresholds ensure that most of the major nuisance insects can be found throughout the year although they typically egress as adults in large numbers at defined points, depending on each species' developmental rate and the prevailing thermal regimes. Wind speed, precipitation and diurnal rhythms also play important factors in governing the egress of flies from filter beds.

Factors Affecting Insect Populations in Filter Beds

7. The medium used in the percolating filters affects species diversity. Factors include both the size and derivation of the substrate used and the smoothness of the substrate and these are variable between WWTWs. Typically, the filter beds are constructed using a rocky substrate, such as gravel, blast furnace slag, clinker, pebbles etc. depending on local availability. Filter beds may be variable in depth, area and shape.

8. Insect populations can be affected by the type and grade of filter substrate used. An important factor appears to be the size of the matrix and it appears that that the major psychodid species favour larger stones, of greater than 5 cm in diameter that generally facilitate higher emergence rates. Similarly, using a smaller grade of medium is also seen as a factor in reducing the emergence of the window gnat, S. fenestralis.

9. The quantity of organic material loaded onto the bed and the presence of industrial and agricultural effluents within the settled sewage applied to the beds can also alter species composition, usually through creating an impoverished faunal structure. For example, T. alternata is favoured by the relatively high loading provided by "strong" sewage. When chemical pollutants are present T. alternata may be the only dipteran species present which, in the absence of competition, can lead to very high population levels.

10. The quantity of film present in a filter is reliant on the dynamics between species present, the extent of scouring and the time of year. Therefore, the availability of food present for developing psychodids may not necessarily be correlated with the physical loading of the bed and competition with other species may become a factor.

11. In the case of S. fenestralis, largest populations occur at high organic matter levels but film levels are quickly reduced resulting in reduction in numbers due to low food availability. Recirculation of effluent to reduce the quantity of organic matter can also reduce populations.

12. Filter beds typically receive a volume of effluent periodically via nozzles on a rotating distributor arm and the volume applied, and the frequency of passes, varies between WWTWs. A low dosing frequency tends to inhibit all dipteran fly populations, increase chironomids but Tinearia alternata and Psychoda species are more prevalent in higher dosing regimes. Also where psychodids are rare, S. fenestralis invasion of the filter bed may occur and may further suppress the filter fly populations.

13. It has been noted that the efficiency of application of sewage to filters is also a factor in species success. Poor distribution over the filter bed leads to drier lanes forming from which egression of Tinearia and Psychoda occurs. Similarly, whilst S. fenestralis larvae are more common in the subjet areas of filter beds, the pupae are more commonly found in the drier interjet regions from where the adults egress. Efficient distribution of liquid over the whole bed, through the installation of splash plates or better jet spacing, allows for a more even wetting of the filter which can suppress fly emergence to a degree.


14. Control of flies deriving from the filter beds of WWTWs is a choice between either a chemical or physical approach. The options available for mosquito control may be broader due to their breeding sites being located away from the treatment process.

15. Whilst chemical control of filter-breeding flies may be necessary in certain circumstances, the damage done to the ecological balance of the beds may lead to reduced efficiency. Extensive studies have shown that manipulation of the filter beds through physical or operational measures can minimize insect nuisance. However, chemical and biological control techniques (such as insect growth regulators) have been investigated although these control measures have not been entirely satisfactory for controlling dipterans.

16. Prior to the advent of synthetic insecticides, a number of chemical techniques involving the application of creosote, paraffin and calcium chloride were employed at WWTWs, primarily for the control of Tinearia alternata and Psychoda species developing in biological filters. However the use of organochlorine insecticides (e.g. BCH, DDT) became prevalent although exposures led to resistance in the target insects and to environmental concerns. Later control measures utilised organophosphate insecticides, such as malathion and pirimiphos-methyl, and whilst effective in some cases, their use was again restricted due to environmental concerns.

17. The benzoyl urea insecticide diflubenzuron (dimilin) has also been evaluated but showed poor efficacy against S. fenestralis whilst against psychodids some activity has been recorded, although not in WWTW applications. A second chitin synthesis inhibitor, cyromazine, has also been explored as a potential midge control agent in the STW environment, albeit with variable degrees of control and in the context of the activated sludge process. More recently, alternative (biorational) methods for the control of insects associated with WWTW filter beds have been evaluated using insect growth regulator ( IGR) and juvenile hormone analogue ( JHA)..

18. IGR has shown potential against mosquitoes in related aquatic environments.

19. Approaches using a JHA-based strategy are particularly attractive as these compounds only act against late stage larvae and thus allow grazing juvenile populations to persist, which is beneficial for the efficient functioning of the filter beds. Further work has involved the use of the entomopathogenic bacterium Bacillus thuringiensis ( Bt). A dipteran specific isolate of this bacillus, Bacillus thuringiensis var. israelensis ( Bti), was tested against several nuisance fly species in both laboratory and field situations, and was shown to be efficacious in reducing numbers of both S. fenestralis and psychodid species, amongst others. The use of strains of this bacterium is typically environmentally benign as it is highly host specific, rapidly kills fly larvae, and the likelihood of non-target effects, particularly to aquatic fauna, is negligible. Currently, Bti is the only larvicide used against insects developing in percolating filters and other aquatic environments associated with WWTWs.

20. For mosquitoes, that breed in still water, a wide range of insecticides, including conventional, biological, botanical and biorational formulations, have been used over the years that include both JHAs, such as methoprene, and Bti formulations. In the case of Bti, a major limitation is the very short window of opportunity for effective use whilst, although it has a broader window for treatment, the efficacy of methoprene cannot be gauged until it is too late to retreat. Bti suffers the additional disadvantage that it is not recycled within insect populations, shows very limited persistence, and efficacy against mosquito larvae has been negatively correlated with organic pollution.

21. Methoprene, on the other hand, has been reported to have better persistence and, in most cases, shows higher efficacy against mosquito larvae. Currently in the UK, as with the filter-derived flies, control at WWTWs is achieved through the use of VectoBac.

22. Many of the suggested methods for physical control have been inferred from observations and research into the biologies of the major dipteran species associated with WWTWs filter beds. These include manipulations of the size of the matrix used in the filter, and by default, the interstitial spaces. Such research has indicated that stone sizes below a certain diameter can be deleterious to T. alternata and Psychoda species resulting in inhibition of adult emergence.

23. However, reduced interstitial spaces can lead to clogging of the filter through build up of organic matter, leading to surface ponding of the sewage. Flooding of filters for periods of time has been considered a potential control for Psychoda species, although periods of 24-48 hrs are typically required to eliminate all filter fly larvae. There is some evidence in the literature that this procedure can be effective, although it requires a watertight filter and the filter bed to be capable of physically withstanding the weight of water held.

24. Conversely, the complete drying of the filter has also been considered, but practicalities limit its potential. Firstly, drying periods may be long and require the filter to be withdrawn from use for periods in excess of a week. Secondly, drying of the filter is severely deleterious to the zoogloea and associated no-dipterans, leading to the beneficial fauna of the filter being effectively destroyed. Enclosure of the filters as a means of preventing the emerged of flies from escaping such as the use of netting has been used particularly due to the withdrawal of available insecticides in recent years.

25. Dosing frequency has frequently been evaluated as a mechanism for regulating the egress of flies from filter beds. This serves both to regulate the wetness of the filter at any given time, and the biological loading. There are indications that the even distribution of sewage over the bed is beneficial is inhibiting filter fly emergence, whist higher organic loading may benefit them. A low dosing rate, in terms of the volume applied has the opposite effect and is frequently cited as a factor in the inhibition of egression of both T. alternata, Psychoda sp. and S. fenestralis adults.


29. The spreading of manure has been associated with a number of fly infestations in recent years, even where it has been demonstrated that the manure used is free from infestation. It is recognised by independent entomologists that the odour given off during spreading can attract naturally occurring populations of flies and causes them to artificially concentrate and increase in numbers.

30. Manure should be incorporated by deep cultivation within 24 hours of spreading. This is in accordance with the DEFRA Air Code 1998 and will minimise odour and ammonia emissions and prevent access by flies that may be in the area.

31. When spreading on pasture, only manure that is free from flies and larvae and of low odour should be used. Animals should not graze fields until the minimum time period recommended by ADAS has passed. Care should also be taken that other DEFRA Codes of Practice for protection of soil and water are followed and that manure is not over-applied. (Spreading of manure that has been stored before spreading will be deemed to be the responsibility of the farmer and not the producer.)

32. Manure should not be applied to ground that is waterlogged, flooded, frozen hard or snow covered. It should not be applied within ten metres of ponds or watercourses or within 50m of wells or boreholes.

33. The spreading of manure on Bank Holidays and Sundays should be avoided.

34. Operators should make every effort to remove mud and manure from the tractor and trailer/spreader wheels before driving on the highway.


Email: Central Enquiries Unit

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