Management of recycled water for sustainable ...

International Journal of Environmental Science and Development, Vol. 1, No. 2, June 2010 ISSN:2010-0264

Management of recycled water for sustainable production and environmental protection: A case study with Northern Adelaide Plains recycling scheme S. Laurenson, A. Kunhikrishnan, N.S. Bolan, R. Naidu, J. McKay and G. Keremane

Abstract— In South Australia, 95,000 megalitres (ML) of municipal wastewater is collected and treated in metropolitan Adelaide. Approximately 50 % of this volume is treated at the Bolivar wastewater treatment plant (WWTP) to produce a high quality wastewater suitable for irrigation without health related restriction to vegetable and salad crops. Following treatment wastewater is piped to horticultural growers on the Northern Adelaide Plains through Virginia Pipeline Scheme (VPS). The establishment of the VPS is not only effective in reducing the amount of wastewater entering the Gulf St Vincent but also facilitates the recycling of otherwise waste water for irrigation purposes. The VPS is the largest recycled water scheme in Australia serving around 250 horticultural growers. This paper provides an overview of the scheme focusing on the level of wastewater treatment at Bolivar WWTP, the value of the treated water as a source of irrigation water, carbon and nutrients for crop growth, and the socio-economic and environmental implications of its use for irrigation. Index Terms—environmental impact, horticultural crops, nutrients, wastewater reuse.

I. WATER ISSUES IN SOUTH AUSTRALIA By global standards Australia is an extremely dry continent with a severe limitation of fresh water resources. A current lack in available water is now the most limiting factor to economic growth within the horticultural sector [1]. In many parts of the country, continued water extraction for agriculture, declining rainfall trends and increased portioning of water for eco-system servicing have led to unsustainable rates of water consumption [2]. In Australia, a total of 24,000 GL of water is consumed per annum for both industry and domestic supply. The vast majority, 65 %, of water is consumed within the agricultural industry for irrigation purposes, followed by domestic supply that consumes 24 %, while mining, manufacturing and service industries consume approximately 11 % [3].

Although agriculture consumes the greatest portion of water, gross value, i.e. economic return per unit of water, is substantially higher in industry relative to agriculture. Values for instance, range from AUS$ 210 ML-1 (Rice) to $ 1,750 ML-1 (vegetables) in agriculture, while in the manufacturing industry, values range from AUS$ 32,000 to $ 680,000 ML-1 and from AUS$ 2,137 to $ 1,000 000 ML-1 in the service industry. A large percentage of surface water in Australia is sourced from the Murray River. This is the largest river in Australia and is of considerable economic, social, cultural and environmental importance. Management of the River is through a consortium of State and local authorizes forming the Murray Darling River Initiative. Under this Initiative, South Australia is allocated 724,000 ML of water per year. As much as 75 % of this allocation is used for irrigation purposes throughout the State and a further 18 % is used to supply metropolitan Adelaide. In an average year Adelaide uses around 216,000 ML of water with approximately 40 % of this amount being sourced from the Murray River while the remaining 60 % is sourced from rain fed catchments in the hills surrounding Adelaide. In dry years however, extraction from the Murray River may supply as much as 80 to 90 % of Adelaide’s total demand so as to compensate for reduced water harvesting from Adelaide Hills Catchments [4]. With an intention to safeguard against water shortages and help restore the health of the Murray Darling River eco-system, current initiatives aim to better manage existing water resources, improve water-use efficiency by both rural and urban users and to better utilize alternative water supplies such as storm water and recycling of wastewaters for irrigation purposes [4].

II. RECYCLED WATER RESOURCES IN AUSTRALIA

Recycled water is a generic term given to water reclaimed from a number of sources, including municipal sewage, Manuscript received June 15, 2010. This work was supported by Co-operative Research Centre for Contamination Assessment and industrial, agricultural and stormwater waste streams. Remediation of the Environment (CRC-CARE). Sewage wastewater is the component of domestic waste S. Laurenson, A. Kunhikrishnan, N.S. Bolan, R. Naidu are with the comprising bathroom and laundry effluents (grey water) and Centre for Environmental Risk Assessment and Remediation of the Environment, University of South Australia, Adelaide, South Australia sewage waste (black water). Generally this wastewater is (corresponding author phone: +61 8 830 26286; fax: +61 8 830 23057; processed through municipal waste water treatment plants e-mail: [email protected]). (WWTP’s) to produce a definable quality of sewage J. McKay and G. Keremane, are with the Centre for Comparative Water wastewater that may be recycled for a range of purposes. Policies and Law, University of South Australia, Adelaide, South Australia. 176


Agricultural wastewater comprises of waste streams from farm and agro-industrial operations. In rural areas of Australia, agricultural drainage effluents and livestock effluents provide a major source of re-usable water [5]. These wastes may also provide a valuable source of nutrients. Industrial wastewater refers to spent water generated from all areas of industrial and commercial enterprise. Some industries may incorporate a certain degree of on-site treatment and/or recycling, however in built-up metropolitan areas such as Adelaide, the vast majority of industrial waste is collected via the municipal sewage reticulation and processed at WWTP’s with domestic wastewaters. Storm water is generally a predominant water source in urban areas. This water source originates from land surface run-off and in many built up areas can contribute a relatively abundant, local source of water during seasons of high rainfall. This resource has been found to have a high level of public acceptance and in many cases storm water harvesting can build upon storm water pollution management facilities, thereby reducing the amount of additional storm water infrastructure required. Annually, large quantities of wastewater are generated in Australia (Table 1), yet only a small proportion is re-used. Around major urban centers, treated municipal sewage wastewater is typically generated in greatest volume, and is used primarily for agricultural irrigation. Extent of re-use ranges from 5.2 % of total volume generated in Northern Territory to 15.1 % in South Australia [6]. A number of successful wastewater irrigation schemes have been developed in Australia, here however we focus on recycling systems in South Australia, in particular the VPS located on the Northern Adelaide Plains. TABLE 1. SOME OF THE MAJOR WASTEWATER SOURCES RE-USED FOR VARIOUS PURPOSES IN AUSTRALIA [3] [6]. States

Recycled water usage in 2004 - 05 (ML) Farm effluents Storm water Sewage

NSW

4471

< 1000

49748

Victoria

23634

< 1000

69651

Queensland

2737

< 1000

44747

Western Australia

2099

< 1000

15263

South Australia

1277

4000

20491

Tasmania

2464

< 1000

4282

Northern Territory

-

< 1000

1852

ACT

-

< 1000

2189

III. WASTEWATER TREATMENT SCHEMES IN SOUTH AUSTRALIA In South Australia, the governmental water authority, SA Water, treats 95,000 ML of wastewater annually through three major WWTP’s servicing around 1.2 million people in metropolitan Adelaide. Bolivar treats approximately 45,000 ML wastewater annually and has resulted in high quality Class A treated wastewater being piped to vegetable growers on the Northern Adelaide Plains. Glenelg treats approximately 21,000 ML wastewater

annually. A high percentage of the treated wastewater is then used to irrigate parks and sports playing fields around Adelaide. Christies Beach treats about 11,000 ML annually and provides treated wastewater to the important wine growing region of McLaren Vale. Recently, as part of the Environment Improvement Program, the Port Adelaide WWTP has been replaced with a new high salinity treatment plant at Bolivar [7]. Two important benefits are gained through the treatment and re-use approach these include (i) reduced the amount of treated wastewater entering Gulf St Vincent and, (ii) recycling of high quality treated wastewater for irrigation purposes.

IV. WASTEWATER TREATMENT PROCESS At WWTPs, wastewater is treated via a multi-stage treatment process prior to being discharged into the environment or re-used. In Australia, this is generally a three stage process involving primary, secondary and tertiary treatment. Primary treatment, the initial stage of the treatment process, is typically limited to physical unit operations that remove solid materials. Initially wastewater will be screened to remove large, inorganic material such as paper and plastics, and then further screened for finer grit and silt particles. Following this preliminary treatment, wastewater is then transferred to primary sedimentation tanks where solid particles of organic material are removed from suspension through flocculation. Primary sludge is allowed to settle out from wastewater through gravity. Although this primary treatment removes a large amount of solids, the treated effluent remains high in biological oxygen demand (BOD), suspended solids and nutrients. The primary treated wastewater then undergoes secondary treatment, a process involving the biological break down of dissolved and suspended organic solids facilitated by naturally occurring micro-organisms. Here, settled wastewater enters aeration tanks or lagoons and is mechanically aerated. The injection of oxygen promotes the growth of micro-organisms and helps to maintain their suspension in the wastewater. During growth and multiplication, the active biomass consumes oxygen and organic pollutants and some nutrient constituents of the wastewater. During this secondary treatment stage, the microbial biomass settles under gravity to the bottom of the tank as secondary sludge. Commonly, under activated sludge systems a portion of the settled sludge is retained in the secondary aeration tanks to maintain a healthy microbial population while the remainder is pumped to anaerobic digesters for further treatment through the ‘solids’ waste stream. The wastewater along with the microbial suspension is then passed into clarification units that remove any remaining microbial 10biomass and suspended solids. Once wastewater has passed clarification, it will then undergo tertiary treatment where disinfectants are used to reduce pathogen numbers that may otherwise pose a risk to

177


human health. Chlorine is commonly dosed into the treated wastewater stream for disinfection. At Bolivar WWTP, wastewater is held for up to 28 days in large shallow ponds where UV sunlight energy and other micro-organisms help to reduce pathogen numbers. Depending on the degree of treatment, a wastewater of definable quality is produced from the wastewater treatment process. In Australia, the level of treatment is generally determined either by the specific risks wastewater discharge may have on receiving environments, or where recycled water is concerned, by the purpose in which it is intended for. Where treated wastewater is to be recycled for purposes such as irrigation of food crops or where close human contact is likely, a higher degree of treatment is required. In South Australia, treated wastewater quality is classified as Class A, B, C or D [8]. When used for irrigation, the level of treatment (i.e. Class) determines the degree of restriction placed on crop type and irrigation method permissible. Recycled water of Class A for example has received a level of treatment greater than classes B to D and is suitable for unrestricted irrigation to all crop and fodder types. The use of class C recycled water however is restricted to a more limited selection of crops. In the case of Bolivar WWTP, the introduction of Dissolved Air Flotation Filtration (DAFF) technology utilizes micro-filtration and floatation to produce a Class A wastewater. This wastewater is then distributed through the Virginia Pipeline Scheme to the Adelaide Plains where it is used for unrestricted irrigation of vegetable and salad crops. Ultimately however, re-use of wastewater for agricultural, industrial and environmental application will need to further consider various factors such level of treatment, cost of delivery and public perception.

V. VIRGINIA PIPELINE SCHEME Commissioned in 1998, the VPS is the first large-scale agriculturally based water recycling scheme in Australia and one of the largest in the world. Historically, the horticulture industry in Virginia has relied on ground water resources for irrigation water supply [9]. As a result of over extraction however, groundwater resources declined to levels thought critically low for maintaining the current rate of horticultural production. Faced with foreseen short falls in water supply, in the 1980’s several horticultural producers began using Class C recycled water pumped from the out-fall channel of the Bolivar WWTP to irrigate market gardens. Essentially growers recognized the potential of wastewater as a new water source that could provide a secure supply for irrigation. This realization, accompanied by the social, economic and the environmental drivers, led to the development of the VPS in 1999 [7].

recycled water from Bolivar WWTP to horticultural farms 35 kilometres north of Adelaide on the Northern Adelaide Plains. Growers utilise approximately 15 GL per year, a volume equivalent to about 16 % of the wastewater generated and processed from all Adelaide WWTPs [10]. Initially, piping infrastructure delivered recycled water to approximately 250 growers across an area of 200 km2. In 2009 however, the pipeline was extended by a further twenty kilometres, thereby providing growers with an additional 3,000 ML of recycled water. To date approximately 18 GL of wastewater from the Bolivar WWTP is recycled for irrigation, thereby reducing the discharge of nutrients and salts to the Gulf of Saint Vincent by 40 % and substantially reducing the demand on groundwater resources across the Northern Adelaide Plains region [11]. Through negotiation, a number of water quality assurances such as a permissible upper limit for salinity, equivalent to 1500 mg L-1 TDS (electrical conductivity ~2340 μS cm-1) were established to meet grower requirements [10]. The DAFF plant commissioned by SA Water in 1999, is instrumental in achieving the high-quality treated wastewater, equivalent to a Class A, enabling unrestricted irrigation of all horticultural crops, including cauliflower, cabbage and potato along with a number of salad crops that are typically eaten raw such as lettuce, tomato and capsicum. Other tree and vine crops such as olives and grapes also receive recycled water irrigation. B. Sustainability issues Planning: When considering the successful integration of recycled water into agricultural and horticultural systems, it is imperative that; (i) optimal productivity of that system be maintained, (ii) any negative impact to off-site environments due to nutrient and salt loss is minimized, and (iii) on-going protection of the soil and surface and ground water quality is ensured [12], [13], [14]. To ensure sustainable irrigation practices, Water Reticulation Services Virginia maintains irrigation management plans that are regularly reviewed and approved by the South Australian Environmental Protection Agency (EPA) for compliancy with environmental legislation. Regulation is coupled with grower information and education programs run by the Virginia Irrigation Association, aimed at promoting awareness of potential effects arising from recycled water irrigation on soils and the natural environment. The Virginia Irrigation Association also undertakes regular monitoring of soils and groundwater. This combination of policies, planning and management with public awareness, support and participation has enabled the VPS to continue operating successfully and has resulted in the economic, social, and environmental stability of the North Adelaide Plains. Socio-economic benefits: Where recycled water is used for agriculture purposes, factors such as production volume and nutrient value of recycled water, land value as affected by recycled water supply, and market access to produce grown using recycled water are considered as sustainability indicators [15]. The current production area under wastewater irrigation in Northern Adelaide Plains is approximately 9,000 hectares consisting of both glasshouse

A. Distribution and use During the development of the VPS, co-operative input was established between three principal parties, SA Waterthe WWTP operators, a private company, Water Reticulation Services Virginia, and a grower’s representative committee called the Virginia Irrigation Association [10]. A 100 km long network of pipelines distributes Class A 178


based and broad acre horticultural industries. Glasshouse industries alone currently support approximately 600 growers and expansion of this sector is increasing by approximately 10 % per year [9]. Agricultural land value in the Virginia Park region has increased two fold over recent years [10] that is thought to reflect a greater degree of water security shared by farmers serviced by the VPS. The fertilizer value of recycled water irrigation has also been acknowledged by growers [16]. Overcoming adverse public perception and maintaining market access for produce grown using recycled water is essential in achieving viability of the VPS [16]. It has been noticed that the acceptance level toward produce grown with recycled water is positive throughout all levels of the retail chain [10]. This has been achieved through communication campaigns aimed at training and educating key stakeholders including industry, retailers, and the public. In addition, wholesalers are regularly kept informed of important developments of the VPS and are given adequate reassurance from growers that product quality is not compromised [10]. Furthermore, endorsement of the VPS by the South Australian Department of Human Services and the South Australian EPA has also helped to build confidence levels amongst consumers. In relation to social sustainability, the commissioning of the VPS has resulted in expansion of the horticulture industry that in turn, has resulted in the creation of more jobs for the region thereby increasing the local labor force [15]. It is also expected that more jobs will be created through development of downstream enterprises such as packing, processing, and marketing [15]. This does suggest a close link between increased horticultural production and greater job opportunities. Social capital describes the value of communication and social networks amongst the members of a society. This can be a powerful mechanism for planners who seek to promote greater equity within cities [17],[18]. Growers and irrigators associated with the VPS encompass a wide range of ethnic and cultural backgrounds [16]. Irrespective of group heterogeneity however, frequent and regular interactions amongst private and public sector and the community as a whole, has helped to establish quality networks [10]. Environmental implications: Some of the major environmental implications of VPS include: (i) reduced use of groundwater for irrigation; (ii) reduced discharge of wastewater to Gulf Saint Vincent; (iii) accumulation of carbon, nutrients and salt in soils; and, (iv) groundwater contamination by nutrients and salt [2],[19]. As indicated earlier, groundwater has been the predominant water

resource for the Virginia horticultural region until the development of the VPS and over exploitation has led to saline water intrusion into aquifers [9]. Greater utilization of recycled water in replace of groundwater in the area has resulted in increased water levels within the aquifers beneath the region and opportunities to further utilize recycled water for aquifer recharge are being investigated [20]. Annually, large volumes of effluent are discharged to Gulf St Vincent from the four WWTPs, resulting in the loading of excessive amounts of nutrients and carbon (Table 2). Discharge of nutrient and carbon rich wastewater to the Gulf degrades water quality and health of aquatic ecosystem. Re-use of recycled water for irrigation across Adelaide is however effective in minimizing the volume of wastewater discharged. The uncoupled nature of water with nutrients and salts in recycled water means these constituents are unavoidably applied to soils with irrigation. Loading therefore directly relates to the volume of irrigation applied. Usually this is determined to meet crop water demands [21]. Annual application of large amounts of salts and excess nutrients may pose a number of adverse environmental hazards and management challenges and will require a degree of management beyond that normally required for conventional water and nutrient sources. The average concentration of nutrients in recycled water varies between Wastewater Treatment Plants and will ultimately affect the total nutrient loadings with irrigation. General nutrient composition of crops, is often used as a guide for fertilizer requirements. Ideally nutrient loading rates in recycled water should aim to meet crop demands. Total nutrient loadings per hectare per season have been calculated, for irrigation rates of 3 to 5 ML/ha (Table 3). These figures suggest recycled water can be a valuable source of nutrients for growers in the North Adelaide Plains. Generally in soils receiving frequent recycled water irrigation, the concentration of sodium and chloride often exceeds that of other macro-nutrients by more than 1-2 orders of magnitude [22]. Excessive amounts of sodium may result in the displacement of other basic cations such as calcium and magnesium with subsequent effect on soil structure [2]. Preferential uptake of sodium at the expense of more desirable cations and nutrients may also affect plant growth [22],[23],[24]. On-going management of soil health and crop quality is therefore integral to the success of the VPS [14],[19],[21]. The irrigation management plan developed by VPS aims to address some of the specific issues that include water and nutrient balances, subsurface drainage and overall irrigation strategy. Although it has been reported that the

TABLE 2. AMOUNT OF TREATED WASTEWATER DISCHARGED AND THE LOADING LEVELS OF NITROGEN, PHOSPHORUS, SUSPENDED SOLID AND BOD TO GULF ST VINCENT IN 2001 TO 2002 [15, [19]. WWTP

Wastewater discharged (ML)

Bolivar 35,957 Christies Beach 8,747 Glenelg 18,894 Port Adelaide 13,731 Total 77,329 ^Port Adelaide WWTP decommissioned in 2004

Wastewater re-used (ML) (% of total)

Nitrogen

477 230 520 456

9,714 (21%) 1,971 (18%) 1,918 (9%) 0.0 (0%) 13,603

131 72 145 67 1,683

179

Phosphorus

Suspended solids

Megagrams 1.981 86 242 235 415 2,544

BOD

108 55 173 55 391


historic use of Class C recycled water from Bolivar WWTP in North Adelaide Plains has not led to any detrimental changes in soil salinity, it has led to significant increase in soil sodium that may increase the potential of soils to disperse [25]. There have been limited reports to date however on the current status of soils in this region since the improved quality of wastewater to Class A. Recycled water also contains high levels of bacteria, viruses and parasites. If not managed appropriately, a heightened risk to human health can curtail the success of a land irrigation scheme [13]. While preventing human infection by wastewater micro-organisms should remain a predominant driver in this industry, these organisms often die quickly once applied with recycled water to soils and persistence in soils tends to be minimal [26]. There is however, some risk of human exposure to these organisms during irrigation. In the case of the VPS, training and awareness programs carried out by the Virginia Irrigation Association, and agreement to pump recycled water in lilac pipes and display of sign boards to warn about the use of recycled water has addressed potential human health hazards [15].

[9] [10] [11] [12] [13] [14]

[15] [16]

[17] [18] [19]

VI. CONCLUSION The VPS is one of the most successful recycled water irrigation schemes in Australia and has enriched local communities on a social, economic and environmental level. The value of recycled water as a source of irrigation water, nutrients and carbon has been well recognized. Success however relies heavily on the on-going minimization of environmental risks associated with nutrient and salt loading with irrigation. Impacts on plant growth and disease transmission also rely on careful water management practices at the farm level.

[20] [21] [22]

[23] [24]

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