Taking control of the situation
Professor Bob Andoh, group technical director for Hydro International, details challenges surrounding current and future combined sewer overflow programmes in the UK
Combined sewers are conduits (pipes, channels, etc) that carry both wastewater (from homes, commercial properties industries, etc) and stormwater runoff when rain falls on urbanised areas. These urban water infrastructure systems have generally been 'out-of-sight, out-of-mind' and have evolved into dumps for practically all waste products other than solid wastes.
With increasing development and urbanisation, combined sewer overflows (CSOs) evolved as relief points on the sewer network to discharge excess wet-weather flows to receiving streams and watercourses as the combined sewer networks became severely overloaded. Since privatisation in 1989, the UK water industry has operated on five yearly asset management cycles and is currently in its third (AMP3: 2000-2005), in which the total planned expenditure on its assets and related services to meet with tighter environmental regulations is of the order of £2B. The major component of the current AMP3 programme in England and Wales relates to the need for the water service companies to rectify 85% of the identified 5,500 or so unsatisfactory CSOs and other intermittent wet-weather discharges by the year 2005 1.
The regulatory requirements for CSOs and other intermittent wet-weather discharges into high-amenity waters, commonly referred to as the AMP2 regulations, calls for these discharges to be free from solids of noticeable sewage origin and other aesthetic pollutants greater than 6mm in two dimensions (2D).
The 6mm in 2D criteria resulted from the enactment of UK regulations in response to the European Urban Wastewater Treatment Directive. Equipment manufacturers responded to this challenge by developing a range of CSO solids control devices and products (mostly screening equipment), some of which have been adapted from conventional forms of screening technology (for example, WwTW inlet screens) and others being novel devices developed specifically for the CSO market 2.screening systems
When the AMP2 regulations were promulgated in the early 1990s, there was very little available by way of combined sewer solids control equipment that had been tried, tested and proven to provide reliable and trouble-free sewer solids control to 6mm in 2D.
A review of the technical literature shows that, with a few exceptions, there has been virtually no reporting on the maintenance and operational aspects and experiences with CSO screening systems. In addition, these systems have not been in place long enough for their long-term reliability and their overall environmental benefits to be established. In the later half of the AMP2 (i.e 1995-2000) period, a programme of screen testing and evaluation was conducted at the National CSO Test Facility at Wigan WwTW by the UK Water Industry Research (UKWIR) CSO Research Group. This work was valuable in demonstrating screening technologies and determining likely sewer solids and aesthetic pollutant removal levels in the field.
The nature and duration of the Wigan test programme, however, did not provide scope for assessing the robustness and the maintenance and operational aspects of these screening systems operating in the unpredictable harsh combined sewer environment. For example, the tests did not assess the ability of the screens to handle flush effects and other issues such as long-term operational performance, durability, maintenance requirements, whole life cycle costs, cost of ownership, etc. Further work was therefore carried out by some of the water companies independently, for example, testing at Knostrop WwTW by Yorkshire Water and field evaluations by Welsh Water 3, to provide the additional information required to assist in equipment selection. Based on the work and experiences gained, three categories of screen types emerged as CSO screening systems that meet with the regulatory requirements. These are:
- non-powered self-cleansing screens.
Broad characterisations can be made of the three categories of screening systems and the particular application areas for which they are suited. Static screens, for example, are the simplest type of CSO screen and are relatively inexpensive. These screens, however, require more operational maintenance than more sophisticated self-cleansing systems. Since static screens generally require cleaning after each spill event, they are therefore more suited to readily accessible sites with low spill frequencies. Non-powered self-cleansing screening systems generally have the lowest whole life cycle costs and are the preferred choice, especially for remote sites and where the hydraulics of the system does not preclude their use. Powered screens on the other hand are more appropriate at congested sites in inner city areas where power is readily available.pollution profile
Combined sewer flows and their resulting uncontrolled discharges under wet-weather conditions are characterised by the presence of faecal and other gross solids (for example, persistent synthetic material - condoms, panty liners, cotton buds, etc); faecal coliforms; total suspended solids; BOD; ammonia; heavy metals (copper, lead, zinc, chromium etc); toxic organics (benzene, phenols and other organic solvents); fertilisers; and pesticides. Other pollutants such as oils and grease may be present depending on the residential, commercial and industrial profile of the sewer network's service area. It is obvious these various constituents will have differing polluting effects when they are discharged into receiving watercourses depending on their quantities, etc. Although a significant proportion of the inert visually offensive substances found in sewage, for example, sanitary products, do not necessarily biodegrade to cause a direct stress on the receiving environment in terms of ecological impact, they are unsightly and often cause an aesthetic pollution problem. The current UK standard for CSOs and other intermittent wet-weather discharges is based around the removal of gross sewer solids and other aesthetic pollutants (for example, sanitary products). This focus on aesthetic pollutants, presumably because this category of pollutants is visual and is what causes public complaints, has in turn resulted in screening becoming the principal technology utilised to address CSO challenges in AMP3 and beyond. The absence of sanitary products (i.e. condoms, panty liners, etc) and other visually offensive pollutants from intermittent discharges, however, does not imply these discharges are free from pollution or do not pose a health risk. Microbial organisms (including pathogens), heavy metals and other toxic micro-pollutants, for example, are attached to sewer sediments.
Toxic micro-pollutants have a chronic effect on fish and other invertebrates and are also dangerous to humans if ingested through the food chain. Polycyclic aromatic hydrocarbons (PAHs), often found attached to sediments, are carcinogenic. Research and operational experiences have shown considerable sediment deposition occurs in most sewerage systems as self-cleansing velocities are hardly attained during dry weather flow conditions.
The bulk of the sediment fraction and their associated pollutants are less than 6mm in 2D and as such are not removed by screening systems. The remobilisation of gross sewer solids, the settled sediments and their associated pollutants during storm events accounts for the observed first-foul flush during storm events, especially after a prolonged period of dry weather.
This highly polluting first flush, including what has been described as the 'fluid sediment' 4, can result in a shoal of detritus with varying amounts of organic pollutants and heavy metals accumulating immediately below intermittent discharge outfalls or where the first slack length of river occurs causing possible ecological impairments, especially during low summer flow conditions.
Despite the above, a worrying trend that has been observed in the UK water industry is the drive towards the use of smaller and smaller CSO chambers built specifically to accommodate screening devices (for example, the recent WaPUG CSO Chamber Design Guide). The geometry of CSO chambers and their resulting hydrodynamic regimes have an impact on their effectiveness in controlling pollutants other than aesthetic solids, including substances in solution and very fine particulates - typically less than 50µm in size.
Depending on their geometry, CSO chambers can be either effective or ineffective sedimentation/mixing chambers, especially regarding the control of sediments and their associated pollutants. Rotary flow chambers, such as hydrodynamic vortex separators, have been found to be more effective than conventional rectangular chambers 5 with smaller chambers generally being less effective in removing and retaining pollutants.
The recent Water Framework Directive from the European Union advocates the need for a focus on maintaining "good ecological status" in the rivers and receiving waters of member countries. If a more holistic approach is taken towards issues of improvements in environmental quality, then achieving water quality objectives, attaining use related water quality standards in receiving waters and minimising potential health risks are all issues that will have to be seriously considered, in addition to the current focus on aesthetic pollutants. In this respect, CSO screening should be regarded as one of the components for controlling the discharge of objectionable sewer solids and pollutants into the environment. Attention needs also to be paid to maximising the effectiveness of CSO chamber designs to provide additional benefits over and above the control of aesthetic sewer solids. Recent advances in computational fluid dynamics (CFD), for example, provide a means of assessing the effectiveness of various chamber configurations and designs in terms of their ability to both remove and retain pollutants 6.
For example, hydrodynamic vortex separation systems with well-configured internal flow modifying components have been shown to be more effective than conventional chambers in terms of their ability to remove and retain sediments, floatable solids and sediment associated pollutants, as well as providing flow regimes conducive to chemical contacting for chemically assisted sedimentation or disinfection to inactivate pathogenic organisms 7.pollutant control
When sewer sediments and attached pollutants are discharged into receiving waters, they settle to the bottom (benthos) in the sluggish regions of receiving waters, creating sludge banks and causing chronic pollution impacts.
Deposits on streambeds also inhibit ecological development and impair the establishment of healthy ecosystems. The organic solids in the benthos cause a delayed oxygen demand (chronic impact) on the overlying waters and the heavy metals cause a progressive build-up of toxicity levels, presenting a potential 'time bomb'. Conventional CSO chamber geometries provide no additional benefits in terms of pollutant control other than hydraulic control for the operation of screens. It has therefore been argued where screens are installed for aesthetic pollutant control, CSO chambers can be made as compact as possible to fit around the screen. This in turn has resulted in a recent trend towards the installation of smaller and smaller CSO chambers designed solely around screens.
In the current climate of increasing regulatory pressure and more stringent environmental standards, this trend towards smaller and smaller CSO chambers would have to be countered by the installation of screens with finer and finer apertures and relatively higher and higher screening efficiencies to avert the risks of reversing the recent gains in environmental quality from the control of point sources such as WwTWs effluents.
Research on the use of finer screens for controlling CSO solids has shown that 4mm is the optimum aperture size for CSO solids control and that at aperture sizes significantly finer than 4mm, for example, 1mm and below, the hydraulic throughput of the screen is significantly reduced compared to that of say a 6mm or 4mm screen, thus necessitating the use of larger CSO chambers to provide equivalent flow capacity 8.
It is the author's view there is the need for a more holistic approach to CSO control in the UK focussing not just on aesthetic pollutants but on the complete spectrum of sewer solids including sediments and their associated pollutants. The UK water industry faces challenges ahead with regards implementation of the new European Water Framework Directive, which focuses on maintaining good ecological status in receiving waters.
The large investment in screening systems to address the current AMP3 programme should result in significant reductions in the quantities of objectionable sewer solids discharged into the environment from CSOs, however, the use of smaller CSO chambers could potentially result in a negative impact with regards to the ecological and bacteriological status of the receiving waters. This is because small CSO chambers do not offer effective control of sediments (and their associated pollutants) and screens do not remove this component of sewer solids. It would be a sad indictment on the UK water industry if in ten or so years from now, the £2B expenditure results in visually pleasing receiving waters that are impaired in terms of other indicators of pollution such as their ecological and bacteriological status lReferences: 1 G Morris (1999). Regulatory Requirements for Storm Sewerage Management. CIWEM Tyne and Humber Branch Conference - Developments in storm sewerage management: Meeting the challenges of AMP3 (unpublished event), June 15, UK. 2 AJ Saul (1998). CSO state-of-the-art review: A UK perspective. UDM 98, Fourth international conference on developments in urban drainage modelling, September, London, UK. 3 A Swift and RYG Andoh (2001). Operation and Maintenance of the Hydro-Jet Screen at 6 Sites in South Wales. WaPUG Spring Meeting, No 17, Coventry, UK. 4 RM Ashley, T Hvitved-Jacobsen, J Vollertsen, T Mcilhatton and S Arthur (1999). Sewer Solids Erosion, Washout and a New Paradigm to Control Solids Impacts on Receiving Waters. Eighth international conference on urban storm drainage, Sydney, Australia, August-September. 5 MC Boner, DR Ghosh and SP Hides (1992). High Rate Treatment of Combined Sewer Overflows in Columbus, Georgia. Water Environment Federation, 69th Annual Conference and Exposition, New Orleans, Louisiana, September 20-24. 6 MG Faram, R Harwood and PJ Deahl (2003). Investigation into the Sediment Removal and Retention Capabilities of Stormwater Treatment Chambers. StormCon, San Antonio, Texas, USA, July 28-31. 7 RM Alkhaddar, PR Higgins and DA Phipps (2000). Characterisation for in-situ disinfection of a hydrodynamic vortex separator (HDVS). Proceedings of the First World Congress of the International Water Association, Paris, July 3-7. 8 RYG Andoh, BP Smith and AJ Saul (1999). The screen efficiency of a novel self-cleansing CSO. Eighth international conference on urban storm drainage, Sydney, Australia, August 30-September 3, pp 2067-2073.