Donata Dubber 1, Laurence W. Gill 1 Corresponding Author Donata Dubber 1

Transcription

1 The suitability of packaged wastewater treatment systems for direct surface water discharge in rural Ireland - A review of performance and cost efficiencies Donata Dubber 1, Laurence W. Gill 1 Corresponding Author Donata Dubber 1 1 Trinity College Dublin, Dublin, Ireland. Abstract Areas with low permeability subsoils are often unsuitable for effluent discharge to ground. Therefore the consented surface water discharge of treated on-site wastewater effluent might need to be reconsidered as a disposal option in such areas. This review shows that packaged wastewater treatment plants are able to achieve high organics, solids, and ammonia removal. Average effluent concentrations are usually lower than the required surface water discharge limits. However, nutrient removal is still limited so that surface water discharge in sensitive areas cannot be considered without further treatment. Enhancing denitrification results in higher process complexity using anoxic zones and effluent recirculation. Chemical phosphorus removal is possible in packaged OSWWT systems but involves expensive, hazardous chemical usage and storage as well as more frequent desludging. Biological TP removal on the other hand is not controllable in automated systems and cannot be considered as an option for packaged systems. The use of phosphorus adsorbing filter material is a promising solution to meet TP discharge limits in sensitive areas. Cost analyses have shown that decentralised systems for clusters of single houses can reduce the per capita cost for operational and capital expenditures compared to single house systems. For all technologies available on the market it should be noted that their reliable treatment efficiency is highly dependent on regular maintenance and protection of user abuses. Thus the on-site and decentralised wastewater treatment with consented surface water discharge can only be environmentally sustainable where appropriate regulations and a suitable management plan are in place. Keywords: on-site wastewater treatment; low permeability subsoils; effluent quality; nutrient removal; surface water discharge; cost analyses. Abbreviations CAS - Conventional activated sludge MBBR - Moving Bed Bioreactor MBR - Membrane Bioreactor OSWWT - On-site wastewater treatment RBC - Rotating Biological Contactor SAF - Submerged Aerated Filter SBR - Sequencing Batch Reactor WWTP - Wastewater treatment plant Introduction The domestic wastewater of over one third of the population in Ireland (approx. 5, dwellings) is treated by on-site systems (CSO, 211) of which the percolation area (soil attenuation system) is an integral part of the overall treatment system. However, where there is insufficient permeability in the subsoil to take the effluent load, surface ponding and runoff of pollutants to surface waters may occur. This represents a serious health risk and can also contribute to eutrophication in sensitive water bodies. The recent specification (EPA, 29) of a lower limit to subsoil permeability (set at T=9) for effluent discharge to ground in conjunction with surface water discharges Page 1 of 8

2 generally not being licensed for one-off housing, means that many areas will be unsuitable for single house development in the future. To allow further development in areas of low permeability subsoils and to achieve the desired river water quality according to European objectives (S.I. No. 272, 29), alternative wastewater treatment and disposal options will need to be considered for new as well as for existing sites. While other appropriate on-site technologies, such as zerodischarge and alternative soil infiltration systems, are being investigated, Local Authorities might also need to reconsider the consented discharge to suitable water courses as a disposal options in areas with low permeability subsoils. For example in areas of relatively dense settlement it could be economically feasible to connect single houses via a small bore sewer system and treat the wastewater at a decentralised plant before consented discharge to a nearby watercourse. There is a large variety of products available on the Irish market so that an overview of all available technologies is needed to be able to select a suitable plant for individual cases. This review will focus on the performance and suitability of available small scale treatment technologies for a direct surface water discharge. Furthermore operational and capital costs of the different systems will be compared. Data collection An extensive online search was undertaken to collect basic information about the available package treatment plants in Ireland. Detailed data on maintenance requirements, treatment performance and costs was then obtained through contacts made with manufacturers and suppliers. According to the Irish Buildings Regulations (DoEHLG, 21) as well as the Code of Practice (EPA, 29) packaged wastewater treatment plants must conform to the International/ European Standard I.S. EN including the relevant national annexes. Therefore the treatment performance of most available systems has been tested at European test facilities to obtain the relevant certificate. These test results have been collected in order to assess the system s ability to meet surface water discharge limits. Results and discussion As the data collection is still in process the presented results should be seen as preliminary findings. Treatment processes used in packaged WWTPs. About packaged wastewater treatment systems were found to be available in Ireland. While 6 plants are only available for larger applications (up to 5 PE), 3 package plants are suitable for the treatment of domestic wastewater from single houses. Figure 1 shows the proportion of treatment processes that are used in packaged wastewater treatment plants in Ireland. With 15 plants the majority of systems use the fixed film process to treat domestic wastewater, while 13 systems are using the activated sludge process and only few media filter and MBRs are available in Ireland (Fig. 1). The 6 systems only available for large scale applications comprise two MBBRs, two MBRs and one SAF and RBC each. A new fixed film system, the Horizontal Flow Bioreactor (HFBR), has recently been developed (Rodgers et al., 26) and will soon be launched into the market as well. packaged treatment plants 3 systems for small and large scale application 6 systems for large scale application only 15 systems using fixed film process 13 systems using activated sludge process systems using media filter 2 systems using membrane technology 11 Submerged Aerated Filters 3 Moving Bed Bioreactors 1 Rotating Biological Contactors 7 Sequencing Batch Reactors 6 Conventional Activated Sludge systems Fig. 1. Proportions of treatment processes used in packaged wastewater treatment plants in Ireland Page 2 of 8

3 Treatment performance and effluent qualities. Over 7% of the available package treatment systems achieved average effluent BOD 5 concentrations of 12 mg/l during testing with SS concentrations usually being below 2 mg/l. Nitrifying bacteria, that convert ammonia to nitrate, are very temperature sensitive so that ammonia removal is dependent on water temperatures within the bioreactor. With decreasing temperatures nitrification can be inhibited. For better comparison of the nitrification potential of packaged treatment plants average ammonia removal rates were determined for temperatures 12 C. Most plants (about 8%) obtained average effluent concentration between.2 and 8 mg NH -N/L. However, consulting detailed performance data from the entire test period showed that even at winter temperatures (reached at the test locations in Germany) ammonia effluent concentrations usually stay within discharge limits. Figure 2 shows the average effluent concentrations for BOD 5, SS and NH -N that are achieved by package plants using different treatment processes. As the data collection has not been completed yet the graph represents performance data from 3 MBBR, 8 SAF, 5 CAS, 5 SBR, 3 filter media, and 2 MBR systems. From the figure it can be seen that the average effluent concentration obtained from most package treatment plants are well below the current surface water discharge limits of 2:3:2 mg/l for BOD 5, SS and NH -N (EPA, 29). In direct comparison it is apparent that the best treatment performance is achieved by plants that use filter media or membrane technologies (Fig. 2). All filter media plants produced effluent with BOD 5 and SS concentrations of 5 mg/l while MBRs reached concentrations of 2 mg/l. No major differences in performance could be observed between the other treatment processes. While 75% of CAS systems achieve generally good BOD 5 removal with effluent concentrations of 1 mg/l, SS concentrations are rather high with a maximum of 23 mg/l being observed as the average effluent concentration in one plant. BOD 5 effluent concentration SS effluent concentration NH -N effluent concentration MBBR SAF CAS SBR Filter media MBR x MBBR SAF CAS SBR Filter media MBR MBBR SAF CAS SBR Filter media MBR Fig. 2. Distribution of average effluent concentrations (BOD 5, SS and NH -N) obtained from package plants using different treatment processes. The boxplot shows the lower quartile (green), the upper quartile (red) as well as minimum and maximum concentrations obtained. However, one CAS plant has the option of an additional filter being incorporated into the system that decreases SS concentrations down to 3 mg/l (indicated as an outlier (i.e. x) in Fig. 2). The MBR systems also achieved the best nitrification rates with NH -N effluent Page 3 of 8

4 concentrations below 1 mg/l (Fig. 2). Due to the membrane filtration solids are contained efficiently within the biological process so that usually long sludge ages and high mixed liquor suspended solids (MLSS) concentrations are reached. These conditions support the slow growing nitrifying bacteria resulting in high nitrification. Filter media systems and MBBRs also achieved good ammonia removal. Most plants produce effluents with 5 mg NH /L. Effluent concentrations from the other treatment processes vary largely between the different plants. Average ammonia effluent concentrations in CAS systems for instance vary from 1 up to 18 mg/l between the different plants. Table 1 summarises the treatment performance and effluent qualities that are obtained from the best performing system of each process type. Table 1. Treatment performance and effluent qualities (according to EN test results) obtained from the best performing system of each process type System BOD 5 SS NH -N TN TP RBC MBBR SAF CAS SBR Filter media MBR < < < for temperatures 12 C effluent concentrations according to manufacturers information * with P-removal by adsorption to granulate = data not accessible /.17* 53.2 / 97.9* Nutrient removal. Several package treatment plants already achieve good N-removal by incorporating anoxic zones; however, removal efficiencies exceeding 65% are rarely achieved. Typically TN-removal efficiencies range between 6 and 65% resulting in effluent concentrations of 12-2 mg/l (Table 1). Only one of the reviewed package treatment plants (Filter media system) has shown that it can meet the discharge limit of 15 mg/l given by the EU Council Directive (91/271/EEC) for the treatment of urban wastewater. No secondary treatment system was able to reach the required limit of 5 mg TN/L (EPA, 29) to allow discharge to surface waters in nutrient sensitive areas. To enhance denitrification effluent recirculation into the anoxic zones is needed. This would allow nitrates, produced in the aerobic stage via nitrification, to be removed from the water. However, this further adds to the systems complexity, increasing costs and maintenance requirements. Phosphate removal is traditionally achieved by chemical precipitation using salts of iron, aluminium or lime as coagulants. While it has proven to be very effective in large scale plants, secure chemical storage and the incorporation of an efficient dosing system could prove difficult for OSWWT systems. Furthermore additional costs for chemicals and more frequent desludging brought about by increased sludge production have to be considered. In order to achieve biological P-removal from wastewater microorganisms are subjected to alternating anaerobic and aerobic conditions. However, this involves a high process control and difficulties in assuring stable and reliable operation have been reported for municipal WWTPs (Blackall et al., 22). Thus, biological P-removal is not a realistic option for automated OSWWT systems. As a consequence most of the package treatment plants do not incorporate any P-removal in their standard models but some suppliers offer chemical P-removal for customised systems. Some package treatment plants achieve up to 5% P-removal even without any chemical dosing. However, discharge concentrations of effluent are still above 3 mg/l (Table 1) so that additional removal is needed if effluent discharge to surface waters in sensitive areas (2 mg/l required) is to be considered (EPA, 29). In past years different phosphorus-adsorbing materials have been extensively examined for Page of 8

5 their P-sorption capacity and their potential use in constructed wetlands or in other small-scale filter systems for TP-removal from domestic wastewater (Westholm, 26). So far there are only few data for performances under field conditions and no empirical data on the longevity of these P-adsorbing substrates, but it is certain that for application purposes it will have to be replaced at some point after saturation. This needs to be considered when these substrates are incorporated into wastewater treatment systems so that it is accessible for the replacement. Furthermore the handling of the P- saturated filter media has to be considered. It can either be disposed at the landfill or find some beneficial use i.e. as fertilizer or soil conditioner in agriculture. Blast furnace slag for instance has been shown to release absorbed phosphorus in a form that is readily available to plants for assimilation (Westholm, 26). However, the use in agriculture might be restricted as toxic pollutant such as metals might be adsorbed to the substrate as well. Based on these concepts a new P-removal filter material will be soon available on the Irish market. The material is reported to not only remove TP down to levels below 2 mg/l but also to reduce the microbial loading of the effluent by 95.5% (as total coliforms removal). However, the disinfection is achieved by an increase of the ph to about 1 so that a ph adjustment will be needed before effluent can be discharged to surface waters. One of the available MBRs can also be supplied with an additional P-adsorption filter bed. However, the manufacturer does not supply the filter separately as a trouble-free operation can only be assured for the treatment of effluent with low BOD 5 and SS concentrations. Impact of higher influent concentrations on treatment performance. With the view to save water and energy costs, or to reduce the hydraulic load to either the subsoil or to water courses, water minimisation devices might become more widespread in rural households. Reducing the wastewater production will proportionally increase concentrations of organics, nutrients and other pollutants which may have an impact on the wastewaters treatability. Table 2 shows concentrations typically expected for domestic wastewater (Metcalf and Eddy, 1991) and how these concentrations will increase with lower water consumptions by using common water saving devices such as dual flush toilets, low flow shower heads and tab aerators. How this will affect package treatment plant performance has not been quantified yet and tests using higher wastewater concentrations may be needed in the future. Table 2. Change of wastewater concentrations with the use of water saving devices Contaminants Typical domestic wastewater concentrations weak medium strong Wastewater concentrations for volume reduced by 2.2% weak medium strong BOD COD TS TSS TN org. N Ammonia TP Cost comparison. Figure 3 shows the plant costs per person when serving a range of population equivalents. For a single house with 5 inhabitants the MBRs ( pp) and MBBRs ( 12 pp) are the most expensive while SAF ( -7 pp) and CAS systems ( 5-55 pp) have the lowest capital costs. While some filter media are in the upper price range ( 1 pp) there are systems that are cost competitive with SAF and CAS plants ( 7 pp). Generally a decrease in capital costs can be observed when larger models are used to serve small communities and with economies of scale per person realised (Fig. 3). For an SAF plant for instance the plant cost per person is reduced by 5% down to 2 for a plant serving compared to 5 PE. Although Figure 3 only shows costs of plants serving up to Page 5 of 8

6 5 PE there are several plants available for small communities of up to PE. Some manufacturers also give the option to design a larger customer specific system outside of their standard range. Per capita plant costs [ ] Population equivalent Fig. 3. Per capita plant cost development for increasing population equivalents served by different package treatment plants SAF, MBBR and CAS systems require an air blower running continuously for 2 hour to supply oxygen to the aeration chamber. For a single household with 5 inhabitants the annual electricity costs for such a system is estimated to range between 2 and 3 per person. An MBR needs an additional vacuum pump for the filtration process so that running costs can be about twice as high. The rotor of an RBC system uses less energy than an air blower so that annual electricity costs are estimated to be around 13 per person in a single household. Figure shows that again for most of the treatment processes, there are economies of scale with respect to per capita operational electricity costs. For an MBBR system for instance running costs can fall from 2 below 1 per person per year. In some filter media systems the effluent is distributed by gravity so that no electricity is needed. Other systems however use a pump with float switch to intermittently apply the effluent over the media. The arising electricity costs to run these pumps are below 5 per person per year. As the pump operates depending on the wastewater production per capita costs are expected to stay similar for larger systems (Fig. ). The aeration in an SBR system does also not run on a continuously basis and treatment cycles are only started when enough wastewater has been collected in the primary chamber. Observed electricity consumptions during the EN testing period has shown that the annual running costs for small SBR plants (up to 8 PE) will range between and 6.6 per person. Per capita electricity costs [ /y] Population equivalent RBC SAF MBBR CAS Filter media Fig.. Per capita electricity cost development for increasing population equivalents served by different package treatment plants Although MBRs are generally more expensive for single houses, one of the systems becomes cost competitive with other systems when used as a decentralised system (Fig. 3). For example, a decentralised plant serving about 225 PE the capital costs are estimated to decrease down to 5 per person. Researchers from the Centre for Water Science at Cranfield University in the UK investigated and calculated the capital and operational expenses of small MBR package plants for domestic use (Fletcher et al., 27). Their results indicated that it is possible to produce a single household MBRs at similar capital costs as one for the more expensive traditional packaged secondary treatment systems although the power requirements would still be times higher for MBRs. However, when using systems designed for more than 2 PE they showed that the cost difference per head becomes negligible. MBR Page 6 of 8

7 Tertiary treatment units. To improve effluent quality, tertiary treatment units such as constructed wetlands, sand filters or other packaged units (e.g. for disinfection or specifically designed nutrient removal systems) can be added after secondary treatment and before the discharge to rivers. These systems can reduce the number of micro-organisms present in the treated wastewater or further decrease organic, solids and nutrient concentrations to achieve standards depending on the sensitivity of the receiving waters. Part 7 of the European Standard deals with "Prefabricated tertiary treatment units, but is only available as a preliminary draft (pren :29). However, it is conceivable that in future these systems can be tested in a similar way as secondary treatment package plants. Maintenance and operational issues. Often the success of a package treatment plant and effective wastewater treatment is dependent on correct installation and regular maintenance of the system. All available plants require at least one service visit per year and need to be desludged regularly. Many suppliers offer service and maintenance contracts that help keeping the system in good working conditions in order to ensure continued treatment efficiency. Significant operational problems could also appear due to user abuse (e.g. disposal of wipes, nappies and greases, or overuse of antibacterial cleaners) resulting in insufficient treatment performance with effluent concentrations exceeding discharge limits. To ensure a successful implementation of advanced secondary and tertiary treatment units for single houses and small communities in rural areas, the correct installation and regular maintenance needs to be enforced by the regulating authorities. Furthermore the risk of user abuse needs to be reduced by appropriate education about wastewater treatment. Conclusions Appropriate treatment technologies are available to meet even strict discharge limits for BOD 5, SS and NH -N. Further developments in TN removal technologies are needed to meet discharge limits for sensitive areas. Additional TP removal is needed to meet discharge limits for sensitive areas. Adsorption filters could be a sustainable solution. The possible increase of influent concentrations due to water saving actions and its effect on the plants treatment performance needs to be considered. Decentralised systems can lower the burden of monitoring associated with discharge consents Decentralised plants can represent an environmentally and economically sustainable solution providing that an appropriate management system is in place to operate and maintain plants. References Blackall, L. L., Crocetti, G., Saunders, A. M. and Bond, P. L. (22). A review and update of the microbiology of enhanced biological phosphorus removal in wastewater treatment plants. Antonie Van Leeuwenhoek International Journal of General and Molecular Microbiology 81(1-): CSO (211). Census 211, Principal Demographic Results. Central Statistics Office, Government of Ireland. Stationery Office, Dublin. DoEHLG (21). Irish Building Regulations Part H - Drainage and Waste Water Disposal. Department of the Environment Heritage and Local Government, Ireland. EPA (29). Code of Practice: Wastewater Treatment and Disposal Systems Serving Single Houses. Environmental Protection Agency, Ireland. Fletcher, H., Mackley, T. and Judd, S. (27). The cost of a package plant membrane bioreactor. Water Research 1(12): Page 7 of 8

8 Metcalf & Eddy, I. (1991). Wastewater Engineering: Treatment, Disposal and Reuse. New York, McGraw-Hill, Inc. Rodgers, M., Lambe, A. and Xiao, L. W. (26). Carbon and nitrogen removal using a novel horizontal flow biofilm system. Process Biochemistry 1(11): Westholm, L. J. (26). Substrates for phosphorus removal - Potential benefits for onsite wastewater treatment? Water Research (1): Acknowledgments The authors like to thank all package treatment plant manufacturers and suppliers who kindly provided detailed information about their systems. Acknowledgments are also given to the Irish EPA for funding this research. Page 8 of 8