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SECTION 10 - GOOD PRACTICE FOR WASTE WATER TREATMENT WORKS
Introduction
1. The major nuisance species derived from STW filter beds are the moth fly, Tinearia (= Psychoda) alternata Say (= 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 when found in, and around, the house. 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 United Kingdom, 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 have been extensively researched and 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 WWWTs 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. Mosquitoes are rarely cited as associated with STWs in the UK. However, the case of the Mogden facility in London, where Culex pipiens form molestus occurs in numbers sufficient to generate complaints from the public, illustrates that they can pose a significant and persistent problem in certain scenarios.
6. The interactions within, and between, macro invertebrate species in WWTW filter beds has been extensively investigated and 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 pest 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.
7. 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 influencing insect species present in filter beds
8. The medium used in the percolating filters has frequently been seen as a significant determinant of species diversity. Factors include both the size and derivation of the substrate used. Physical characteristics, such as smoothness of the substrate have also been seen as a factor and are variable between WWTWs. Typically, the filter beds are constructed using a rocky substrate, such as gravel, blast furnace slag, clinker, pebbles etc. The use of such materials is typically determined by local availability and filter beds may be variable in depth, area and shape.
9. Insect populations have been widely reported to be affected by the type and grade of filter substrate used. An important factor appears to be the size of the matrix, although taking single factors in isolation can be misleading. This notwithstanding, it appears that that the major psychodid pest species favour larger stones, of greater than 5 cm in diameter, that generally facilitate higher emergence rates of the imago. Similarly, using a smaller grade of medium is also seen as a factor in reducing the emergence of the window gnat, S. fenestralis.
10. The quantity of organic material loaded onto the bed can have a marked impact on the invertebrate populations present within a given filter bed. Furthermore, the presence of industrial and agricultural effluents within the settled sewage applied to the beds can also manifestly 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.
11. However, the quantity of film present in a given 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.
12. In the case of S. fenestralis, largest populations occur at high organic matter levels but film levels are quickly reduced. As a consequence of the removal of film, numbers subsequently become severely limited by low food availability, a scenario that frequently occurs following high population levels of this insect at certain times of the year. However, recirculation of effluent to reduce the quantity of organic matter can also reduce populations.
13. Filter beds typically receive a volume of effluent periodically via nozzles on a rotating distributor arm. Various types of distribution mechanisms are used. The volume applied, and the frequency of passes, varies between WWTWs and can manifestly affect filter efficiency as well as species richness and the outbreak of a given insect as a pest. It has been indicated that a low dosing frequency inhibited all dipteran fly populations when compared to higher dosing rates. Importantly, dosing rate was also seen to affect which species were present, with Tinearia alternata and Psychoda species more prevalent in higher dosing regimes, whilst at lower dose rates chironomids were more prevalent. There is also evidence that, where psychodids are rare, S. fenestralis invasion of the filter bed may occur and may further suppress the filter fly populations.
14. It has been noted that the efficiency of application of sewage to filters is also a factor in species success. For example, den Otter demonstrated that 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 for example, allows for a more even wetting of the filter which can suppress fly emergence to a degree, although Hawkes suggests that better spacing of jets, without the use of splash plates, constitutes a better distribution mechanism.
Control
15. Control of flies deriving from the filter beds of WWTWs can largely be seen as a choice between either a chemical or physical approach. The options available for mosquito control, however, may be broader due to their breeding sites being located away from the treatment process.
16. Whilst chemical control of filter-breeding flies may be necessary in certain circumstances, the damage done to the ecological balance of the beds is undesirable and may lead to the build up of organic matter within the filter and a reduction in faunal diversity leading to reduced efficiency. Many of the extensive studies into the biology and ecology of the major insect species associated with WWTWs have yielded findings that allow for the manipulation of the filter beds in such a way as to minimize insect nuisance occurrence through physical or operational means. Despite this, chemical control techniques have been widely explored and, more recently, biorational techniques using biological control agents and insect growth regulators have been investigated. However, despite there being a number of control strategies available, it has generally been thought that none of the control measures that have been implemented for the control of dipterans that develop in percolating filters have been entirely satisfactory.
17. Prior to the advent of synthetic insecticides, a number of chemical techniques were employed at WWTWs, primarily for the control of Tinearia alternata and Psychoda species developing in biological filters. These techniques used the application of creosote, paraffin and calcium chloride to the filter beds. However, during the 1940s, the use of organochlorine insecticides (e.g. BCH, DDT) became prevalent and showed some degree of efficacy although exposures rapidly 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, particularly regarding non-target toxicity.
18. The benzoyl urea insecticide diflubenzuron (dimilin), a compound that inhibits the synthesis of chitin, a component of the insect cuticle, 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 STW filter beds have been evaluated. Coombs et al. reported trials using the insect growth regulator ( IGR) pyriproxyfen, a compound that mimics the activity of an insect's own endogenous juvenile hormones ( JH). This JH analogue ( JHA) was seen to provide a measure of control of T. alternata for periods of approximately one month at a Manchester STW, with no adverse effects observed in non-dipteran fauna.
19. This IGR has similarly shown potential against mosquitoes in related aquatic environments. Similarly, other workers have evaluated a second JHA, methoprene, against T. alternata and, although they provided no data on efficacy when applied to filter beds, showed that control of filter flies could be achieved in some scenarios and not in others. A methoprene-based product specifically for midge/filter fly control in filter beds is available in the USA as the formulation "Strike".
20. Approaches using a JHA-based strategy are particularly attractive as these compounds only act against late stage larvae and, as a result, 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. Research in Japan has indicated that several other Bt isolates may also have potential against a range of nematocerans, including filter flies. 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 (available as the formulation VectoBac) used against insects developing in percolating filters and other aquatic environments associated with WWTWs.
21. 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.
22. 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.
23. 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 STWs 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.
24. 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 have 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.
25. 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 was first attempted prior to World War II as a means of preventing the emerged of flies from escaping although cost and other factors largely limited the uptake of this method for the prevention of insect nuisance. Withdrawal of available insecticides has, however, led to widespread use of netting as a means of physically enclosing filter beds in the UK in recent years.
26. 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. The evidence, however, is somewhat contradictory as to which regime is best although 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.
Summary
1. Filter bed facilities are the major cause of insect nuisance from WWTWs across the UK. Despite being gradually phased out in favour of ASP facilities, at least half WWTWs in the UK continue to use trickling filters.
2. Minor problems are reported to be associated with nuisance insects emanating from WWTW filter beds.
3. Window gnats, filter flies and chironomids are the primary causes of nuisance.
4. A major problem is the location of new housing and/or business premises in close proximity to WWTWs
5. Procedures associated with the location of pest sources, and the identification of the pest species appear to be, for the most part, adequate.
6. There is a requirement for an effective adulticide that can be applied when large numbers emerge from the filters.
7. Breeding sites for mosquitoes (i.e. stationary water) should be prevented or removed.
8. Insect pest control at WWTWs should be proactive and undertaken routinely as part of the usual operation of the plants and not solely in response to complaints.
9. Simple screening measures should be implemented where feasible (netting, enclosure) and the planting of barrier vegetation (tall trees/shrubs).
10. The development/investigation of effective adulticides should be considered to alleviate problems when larvicides and physical control procedures prove insufficient to prevent the egress of large numbers of adult flies.
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