Urban wastewater

Stormwater and treated sewage from Australia's coastal towns and cities is discharged to rivers, estuaries, nearshore waters and beaches. The first State of the Marine Environment Report, or SOMER (Zann 1995), identified coastal waters adjacent to major coastal population centres as major pollution 'hotspots', which have been affected by discharges of nutrients, pathogens, heavy metals, sediments, industrial wastes and rubbish through the urban wastewater systems.

Table 3.2 is indicative of wastewater pollution which has occurred near many major urban centres. The significance of this pollution depends on residence time, water circulation and the severity of impact on life forms in the receiving area.

The majority of Australian urban areas have wastewater treatment plants. A very few towns do not, for example Swansea in New South Wales, which pipes untreated wastes to the sea. There is a wide variety of technologies available for the treatment of effluent, and their effectiveness varies with cost. The description of these technologies commonly refers to primary, secondary and tertiary treatment of wastes. Primary treatment involves removal of all large solids (screening) and some suspended solids. Secondary treatment removes the remaining gross pollutants and organic matter by bacteria in 'activated sludge' or a 'trickling filter'. Tertiary treatment involves filtering, disinfection and nutrient stripping. Metropolitan treatment plants mainly discharge secondary treated wastes to the marine environment, usually a few hundred metres offshore, or to nearby rivers and estuaries. Since 1990, most of Sydney's sewage has been delivered 5 km offshore to the continental shelf by deep ocean outfalls at North Head, Bondi and Malabar. Woodmans Point treatment plant in Perth discharges some 3 km offshore (see page 102).

Stormwater is normally discharged straight to a river, beach or ocean, taking with it solutes and solids from roofs, paths, streets and cultivated land in accelerated and enlarged flows. Not only have city and agricultural pipes

Table 3.2 Pollutants discharged in wastewater in Australia

reduced storage and infiltration, but also many natural wetlands, where floods would formerly soak away or evaporate and silt would accumulate, have been 'improved' out of existence.

As a diffuse source of pollution with a great variation of concentration, there has been much uncertainty over the importance of stormwater to the pollution load of nearshore waters. This uncertainty is being reduced by the considerable research activity currently under way or recently completed. This is driven by public concern over the pollution of beaches and nearshore waters: investigations undertaken recently at both Sydney and Melbourne illustrate this.

The Sydney Region had a population of approximately 4 million at the end of the 20th century. Extending 50 km from Emu Plains to Sydney Heads and 140 km from Wyong to Appin, it is the least densely populated major world city. For many years there has been strong public concern about the pollution of Sydney's beaches and bathing waters. This large area is almost completely sewered and has established piped storm drainage to rivers and directly to the coast: there are 200 major (450 mm) storm drains discharging to coastal waters between Palm Beach and Cronulla (NSW EPA 1993). In much of the metropolitan area sewage pipes overflow into stormwater systems at times of flood, and the sewage is then transported untreated to the sea. Sydney Water (1995) noted that of the region's 3000 sewer overflow points, 200 were reported to be overflowing even in smaller storms. A study by the Sydney Water Board (1992) showed that, in three of five small Sydney coastal catchments (Bondi, Whale Beach and Greendale), dry weather faecal coliform counts exceeded the NSW Clean Waters Act standards for primary contact on 98% of occasions, and that nutrient levels were also high. The Sydney Water Board has undertaken investigations on stormwater in a number of catchments. The NSW EPA (1993, p. 43) noted: 'The information to date suggests that, in highly developed areas such as Port Jackson and the Georges River, the main contributor to nutrient and sediment load is stormwater. Water Board results indicate that, while nutrient loads from stormwater and sewage sources vary between catchments, in Port Jackson the split has been found to be 60% from stormwater and 40% from sewage overflow.'

Much concern in Sydney has been about the cleanliness of recreational beaches and waters, but recent investigations in Melbourne have had a different focus. The CSIRO Port Phillip Bay Study (Harris et al. 1996), funded by Melbourne Water, has examined the state of the whole of the Bay waters, all the inputs to it, material and energy flows, life forms and productivity. There is small likelihood that the high-energy, open ocean Sydney coastal waters could suffer nutrient build-up because of water circulation, but deterioration of Port Phillip Bay waters has been a matter of concern, because of its enclosed nature. Port Phillip Bay receives stormwater run-off from a large, intensively cultivated agricultural area and a metropolis of 3.4 million people, as well as treated sewage from half those people, yet the study demonstrates that the Bay shows few signs of eutrophication. The waters and floor of the Bay have a wide variety of flora and fauna and, except adjacent to limited pollution sources, the waters are clear and healthy. Although the sewage treatment works and the Yarra River (principally) deliver large quantities of nitrate and ammonia to the bay, nitrogen (N) does not accumulate. This is not achieved by tidal flushing, since changeover time for water between the bay and the ocean is one year; rather, it is accomplished by denitrification by bacteria of the microplankton and microphytobenthos within the Bay's floor sediments. 'The single most important discovery of the Port Phillip Bay Environment Study (PPBES) is that denitrification in the sediments prevents soluble and available N (ammonia and nitrate) from accumulating in the water column because it is removed from the main Bay environment as nitrogen gas (N2)' (Harris et al. 1996).

Nitrogen is the limiting factor for algal growth in the Bay. It is thus the particular dynamics of the Bay ecosystem that has allowed it to cope with raised nitrogen inputs without the response of excessive algal growth. Rubbish and bacterial pollution near stormwater outfalls is present on Melbourne beaches, as at Sydney, but it is not a matter of such strong public concern. In response to the concern expressed over the state of the Port Phillip Bay waters the CSIRO study has demonstrated conclusively that the Bay can cope with raised nutrient inputs at or near their present level. However, exotic ballast-water hitchhikers, such as the sabellid worm Spirographis Spallanzani, threaten the stability of the benthic ecosystem which is vital to the dynamics of the Bay.

A generation ago, any concerns with stormwater focused only on the adequacy of the system in dealing with urban floods. Today this has changed to concerns over water quality and conservation of stormwater for storage and reuse. State, territory and Commonwealth agencies, local governments, and universities are now undertaking a multitude of stormwater projects and applied research to address these concerns throughout Australia. The Coasts and Clean Seas component of the Commonwealth Coastal Action Program has, since 1996, been a significant funding source in this area, as well as a stimulus to innovation through its criteria.

There are a number of chronic problems in dealing with urban wastewater:

• the need to modify large and expensive urban pipe systems

• uncertainty over relative impacts of stormwater and sewage and the links in the chain of changes that follow pollution

Table 3.3 Some examples of contemporary stormwater projects and research

• public concern over pollution is not matched by a realisation that the causes are related to individual actions and choices

• the belief among agencies and governments that the cost of addressing the problem is too great as compared with the benefits that would flow from a more comprehensive attack on the problem.

The issue of wastewater illustrates the interconnectedness of a wide range of activities and responsibilities involved in coastal management, as well as the need for both a flexible definition of the coastal zone and the need for a means of setting common priorities across a number of agencies. Within the present Australian context, public concerns over coastal water pollution are marching together with concerns over the loss of usable water. Much water that presently runs to the sea (often after expensive treatment to potable level) could be recycled through wetlands, treatment plants or groundwater for reuse. The list of examples of recent or current research in table 3.3 points to changes in ways of thinking about wastewater going on all around the continent.

This change in perception is not only beginning at the institutional level; it is beginning to appear at the community level in a growing realisation that what goes down the drain in the suburban street or from the workshop, may well end up on the beach – see the 'Save our Surf' report of the Surfrider Foundation (Wilkinson 1996). Campaigns, community and council stormwater guidelines and water-sensitive urban design are vital to the health of coastal waters around cities. The extensive replacement of an urban stormwater infrastructure that was designed to use the ocean as a sump does not at this time appear feasible: it is possible, however, that much more water could be retained on suburban blocks, in roadside swales, in parks, and in the soil under porous paving.