Impact of the Upgrade of Cronulla Sewage Treatment
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Impact of the Upgrade of the Cronulla Sewage Treatment Plant on Water Quality at Sutherland Beaches Courtney May, Brad Crossman and Fiona McPherson Beachwatch Programs, Department of Environment and Conservation, PO Box A290, SYDNEY SOUTH, NSW 1232. Beachwatch@environment.nsw.gov.au Keywords: beach, sewage, swimming, water quality Abstract The Sutherland beaches attract more than 1,000,000 visitors each year. Prior to 2001, the beaches were among the most polluted in the Sydney region, with elevated levels of indicator bacteria recorded in both dry and wet weather conditions. The main source of pollution was effluent discharged from the Cronulla Sewage Treatment Plant at Potter Point on the Kurnell Peninsular. The plant was upgraded in 2001 to include tertiary treatment and ultraviolet disinfection. As a result of the upgrade, recreational water quality at the Sutherland beaches improved considerably and they are now among the cleanest in Sydney. This study also highlights the importance of the bacterial indicator enterococci in assessing recreational water quality in marine waters. Introduction With changing community awareness, and an increasing focus on amenity values of public space over the past three decades, there has been an ever-growing demand in Sydney for a high standard of water quality at swimming locations. This is driven not only by the desire for safe swimming conditions to exist but also general recognition that anthropogenic impacts on the environment are affecting ecosystems as a whole, and ought to be ameliorated. The Department of Environment and Conservation’s (DEC) Beachwatch Program began in 1989 and now monitors recreational water quality at 130 locations in the Sydney, Hunter and Illawarra regions, including eight ocean beaches in the Sutherland Shire (DEC, 2005). Beachwatch Programs monitors bacterial levels at swimming locations and provides the community with information on the pollution status of beaches to enable informed decisions about where and when to swim to be made. This information not only identifies immediate patterns in water pollution but also allows trends in water quality through time to be identified and investigated. The Sutherland Shire ocean beaches are situated around the rim of Bate Bay, approximately 18 kilometres south of the Sydney Central Business District. At the northern end of the bay a continuous strip of wide sandy beach links Boat Harbour and the lifeguard-patrolled areas Greenhills, Wanda, Elouera, and North Cronulla. At the southern end of the bay, near the entrance to Port Hacking, are three smaller beaches in rocky coves; South Cronulla, Shelly and Oak Park (Figure 1). The Sutherland beaches attract more than 1,000,000 visitors each year (Sutherland Shire Council, 2005). Pollution sources into Bate Bay include numerous stormwater drains that discharge onto the beach, potential leachate from the former landfill site at Wanda Reserve, estuarine plumes from Port Hacking during wet weather, and treated effluent discharged from the Cronulla Sewage Treatment Plant (STP) at Potter Point (Figure 1). The Cronulla STP was constructed in 1952 and was the fourth plant to be built in the Sydney region (Sydney Water Corporation, 2002). In 1999, work commenced on a substantial upgrade to the STP to provide tertiary treatment and ultra-violet disinfection of effluent to decrease the concentration of nutrients and microorganisms. The upgrade was completed in April 2001 and the STP is now one of the largest tertiary treated STPs in the world, servicing a population of 224,000. The plant discharges 54 ML of tertiary treated effluent to the ocean each day. Figure 1: Location of Sutherland beaches, the Cronulla Sewage Treatment Plant and the outfall. While the STP is a major potential source of pollution to the bay, the southern part of Bate Bay is densely urbanised and there are numerous stormwater drains that discharge onto the beaches. As well, leachate from a former landfill site at Wanda Reserve is a potential pollution source. Estuarine plumes from Port Hacking are also thought to impact the area following heavy rainfall. This paper describes the impact of the upgrade of the Cronulla STP on recreational water quality at Sutherland beaches. Methods Monitoring and reporting of recreational water quality by the Beachwatch Program is based on, but not identical to, the National Health and Medical Research Council (1990) guidelines (NHMRC, 1990). These guidelines have recently been revised (NHMRC, 2005), but have not yet been adopted in New South Wales. Sample collection A water quality sample is collected from the eight Sutherland beaches every six days by Sutherland Council lifeguards. The time of sampling and any visual signs of pollution are also recorded. Samples are collected from between the flags at patrolled beaches and in the centre of unpatrolled beaches at a depth of 30 centimetres. Samples are collected using aseptic sampling in 250 mL polycarbonate containers and are stored on ice. Samples are delivered to a commercial laboratory for analysis within eight hours of collection. Indicator bacteria The two bacterial groups recommended by the NHMRC 1990 guidelines are thermotolerant coliforms and enterococci, both of which are monitored by Beachwatch Programs. Samples from the Beachwatch Program are submitted to an independent NATA accredited laboratory, which analyses the samples using membrane filtration based on APHA (1998) Standard Methods. Beachwatch water quality guidelines Beachwatch considers that waters are unsuitable for swimming if: • the median faecal coliform density exceeds 150 colony forming units per 100 millilitres (cfu/100 mL) for five samples taken at regular intervals not exceeding one month, or • the second-highest sample contains equal to or greater than 600 cfu/100 mL (faecal coliforms) for five samples taken at regular intervals not exceeding one month, or • the median enterococci density exceeds 35 cfu/100 mL for five samples taken at regular intervals not exceeding one month, or • the second-highest sample contains equal to or greater than 100 cfu/100 mL (enterococci) for five samples taken at regular intervals not exceeding one month. Beachwatch uses a rolling median (and rolling second-highest sample) to calculate seasonal compliance for each beach (DEC, 2005). This method has the following advantages: • it overcomes artificial boundaries between months • it provides a greater number of values for interpretation • results are more responsive to, and representative of, environmental events. Seasonal compliance provides the community with a simple and relative measure of beach cleanliness, which can be compared between beaches and between years. The percent seasonal compliance with recreational water quality assessment criteria is calculated as follows: Percent compliance = (number of rolling 30-day periods passing criteria ÷ total number of 30-day periods in the season) × 100. Results and Discussion Compliance levels Using the seasonal compliance index developed by Beachwatch, recreational water quality guidelines for each summer season between 1994–1995 and 2004–2005 are presented in Figure 2. The results are further condensed and are calculated as the mean compliance of all eight Sutherland beaches for thermotolerant coliforms and enterococci (Figure 2). 98 99 98 99 100 97 100 100 100 96 95 96 94 94 94 93 80 Percent compliance 69 58 60 53 55 51 40 23 24 20 0 1994-95 1995-96 1996-97 1997-98 1998-99 1999-2000 2000-01 2001-02 2002-03 2003-04 2004-05 Thermotolerant Coliforms Enterococci Figure 2: Average percent compliance of Sutherland beaches with Beachwatch recreational water quality guidelines between 1994–1995 and 2004–2005. The average compliance with the thermotolerant coliform criteria has remained relatively constant over the last ten years, generally remaining above 93%. These results indicate that discharges from the Cronulla STP had little impact on faecal coliform levels at Sutherland beaches. In contrast, the average compliance with the enterococci criteria increased substantially in summer 2001-2002, following the upgrade of the Cronulla STP. In the years prior to 2001–2002, average enterococci compliance for Sutherland beaches was generally less than 60% and as low as 25%. In the years since the upgrade, enterococci compliance has been similar to that for faecal coliforms, generally 94% or more. The reason why an impact was not apparent for thermotolerant coliforms, but was apparent for enterococci, is likely to be related to the viability of the two bacteria in marine waters. Thermotolerant coliforms generally only survive in marine waters for between 24 and 48 hours (Gerba et al. 1979 and Vivian 1986). So while they are excellent indicators of recent faecal contamination, they are poor indicators of contamination that may have occurred more than two days prior. As the Potter Point outfall is located several kilometres from the Sutherland beaches, it is likely that the majority of thermotolerant coliforms died before reaching the sampling locations. Enterococci are able to tolerate the salt levels of marine waters for longer periods and so are able to indicate the presence of residual sewage contamination. As pathogens in sewage, especially viruses, are also generally able to survive marine conditions for long periods, enterococci play a very important role in the assessment of recreational water quality of marine waters. Epidemiological studies conducted in recent years have highlighted this importance by showing that enterococci levels are better at predicting illness rates in bathers in marine waters than thermotolerant coliforms (Pruss, 1998). As a result of these studies, the new NHMRC guidelines recommend the measurement of enterococci in marine waters (NHMRC, 2005). Response to rainfall In general, faecal contamination at ocean beaches increases with rainfall, with the specific response at each beach dependant on factors such as the catchment area, level of development and the condition of the sewerage system. Beachwatch provides the community with daily advice on pollution levels at Sydney’s beaches. As it takes up to 48 hours to analyse water samples for indicator bacteria, the advice is based on rainfall levels in the previous 24-hours. At the Sutherland beaches, a rainfall threshold of 15-mm is used. When daily rainfall exceeds this level, the bulletin indicates that there is likely to be pollution at the beaches. Prior to the upgrade of the Cronulla STP, the relationship between rainfall and bacterial levels at Sutherland beaches was generally poor. Figure 3 shows the response of bacterial levels at Shelly Beach to daily rainfall levels measured at Cronulla STP between 1998 and 2000. Thermotolerant (faecal) coliform levels tend to increase with increasing rainfall, and generally remain below the median guideline limit (150 cfu/100 mL). In contrast, enterococci densities remain relatively high in all rainfall categories (mostly above the 35 cfu/100 mL median guideline threshold), with an increase only apparent after more than 20-mm of rainfall is recorded in the previous 24-hours. 10000 10000 Faecal Coliforms Enterococci Bacterial count /100 mL 1000 1000 100 100 10 10 1 1 0 0 0 - 5 5 -10 10 -20 20+ 0 - 5 5 -10 10 -20 20+ 24hr Rainfall (mm) 24hr Rainfall (mm) Figure 3: Response of thermotolerant (faecal) coliforms and enterococci at Shelly Beach to daily rainfall levels measured at Cronulla STP between 1998 and 2000. Dashed lines are median guideline limits. As a result of the poor relationship and the generally high bacterial levels in all rainfall categories, Beachwatch was unable to predict pollution levels at Sutherland beaches with accuracy using rainfall in the previous 24-hours. Therefore, until 2001 the Beachwatch daily bulletins carried a specific message for Sutherland beaches warning of pollution from Cronulla STP in both dry and wet weather conditions. Since the upgrade to the Cronulla STP, the relationship between bacterial levels and rainfall at Sutherland beaches has improved, particularly for enterococci. Figure 4 shows the response of bacterial levels at Shelly Beach to daily rainfall levels measured at Cronulla STP between 2003 and 2005. Thermotolerant (faecal) coliforms increase slightly with increasing rainfall, but mostly remain below the median guideline limit (150 cfu/100mL) in all rainfall categories. Enterococci levels also clearly increase with increasing rainfall, occasionally exceeding the median guideline limit (35 cfu/100 mL) after 10 to 19.9-mm of rainfall in the previous 24-hours and regularly doing so after more than 20-mm of rainfall in the previous 24-hours. 10000 10000 Faecal Coliforms Enterococci Bacterial count/100 mL 1000 1000 100 100 10 10 1 1 0 0.1–4.9 5–9.9 10–19.9 20+ 0 0.1–4.9 5–9.9 10–19.9 20+ 24-h rainfall (mm) 24-h rainfall (mm) Figure 4: Response of thermotolerant (faecal) coliforms and enterococci at Shelly Beach to daily rainfall levels measured at Cronulla STP between 2003 and 2005. Dashed lines are median guideline limits. With the improved water quality and the greater accuracy in predicting poor water quality at Sutherland beaches, the message warning of pollution from Cronulla STP was removed from the daily bulletin. Conclusions While the Potter Point outfall is not the sole source of pollution to beaches of the area, there was a dramatic improvement in the compliance of all the sites nearby with enterococci guidelines that followed the upgrade of the plant to tertiary treatment in 2001. It seems certain that the reduction in indicator bacteria is due to the higher standard of effluent being delivered to the waters at the north end of Bate Bay after the upgrade. The Sutherland beaches, which were previously some of the worst performing in the Sydney region, and are now among the best. The upgrade has also revealed a good relationship between bacterial levels and rainfall at the Sutherland beaches, allowing Beachwatch to provide more accurate predictions of pollution levels on a daily basis. This study has also highlighted the importance of the bacterial indicator enterococci in assessing recreational water quality in marine waters. Due to its longer survival time in salt-water conditions, enterococci is able to detect the presence of aged sewage contamination, such as that which would occur when a pollution source is distant from a beach. References Department of Environment and Conservation, 2005 Beachwatch and Harbourwatch Annual State of the Beaches Report 2004–2005. Department of Environment and Conservation, Sydney. Gerba, C P, Goyal, S M, LaBelle, R L, Cech, I. and Bodgan, G F (1979). Failure of indicator bacteria to reflect the occurrence of enteroviruses in marine waters. American Journal of Public Health, 69: 1116–19. NHMRC (1990) Australian Guidelines for Recreational Use of Water, Australian Government Publishing Service, Canberra. NHMRC (2005) Guidelines for Managing Risks in Recreational Water [available at: http://www.nhmrc.gov.au/publications/synopses/eh38.htm - accessed 21/10/05]. Pruss (1998) A review of epidemiological studies from exposure to recreational water. 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