The storage of biosolids, the solid fraction of sewage waste, is a contentious subject for any developed area. Fresh biosolids or oxidation pond sludge waste has a water content of approximately 70% by weight. When disposed of on land, natural drying will generate a crust of approximately 15 cm; below 15 cm minimal drying will occur. Mechanical drying of the fresh waste, is possible, but is costly and is certainly not applicable to previously deposited volumes of waste. Every major populated area in New Zealand and Australia has large volumes of unstable, semi-liquid waste stored on potentially valuable land.
Melbourne Water is the largest water utility in Melbourne, with a sewage system of 380 km and two water treatment plants, the Western and Eastern Treatment Plants. Melbourne produces on average 900 million litres of sewage a day, 54% of which is treated by The Western Treatment Plant (WTP) situated on the western shore of Port Philip Bay near the city of Geelong. The WTP covers 11,000 ha of valuable land, and discharges approximately 600 litres of treated water daily into Port Philip Bay. Three treatment methods are used for incoming sewage. An extensive lagoon system is used for peak daily and year-round wet weather flow. Land filtration is used during periods of high evaporation between October and April. Grass filtration is used during periods of low evaporation between April and October. Extensive land contamination with organic and inorganic contaminants has occurred at the WTP as a result of sewage disposal practices over the past 100 years. Commercial and governmental groups in Melbourne have developed a ‘Vision for Werribee’, a long-term plan that aims to turn solid and liquid sewerage waste into valuable revenue streams, and that will release decontaminated land to capital development. The Vision for Werribee brings together several environmental technologies to achieve the desired objective. One of these technologies is phytoremediation.
We investigated the potential use of plants to dewater the stockpiled biosolids. Biosolids have potential use as a soil amendment or energy source (as a renewable ‘brown coal’), however, high water content limits this potential. For the period between 1973 and 1979 (the only period when climatic data was collected), average annual rainfall at Werribee was 641 mm, while average annual evaporation for this period was 1386 mm (Fig. 1). For no month did rainfall exceed evaporation and this indicates that if water can be removed using plants then no rainfall-induced recharge should occur. An experimentally derived biosolids water-retention curve shows that, in theory, the water content could be lowered to approximately 20% (10 bars) by plant transpiration. Interpretation of this physical and climatic data suggests that plants should be able to dewater the Werribee biosolids. Chemical analysis of the biosolids indicated that elevated concentrations of Cu could possibly affect plant growth, however no signs of phytotoxicity were observed during experimental work.
A one-year demonstration trial was initiated in May 2002 on a biosolids storage tank. The relative growth performance of ten plant species was tested during the trial. Core samples (0-20 cm) collected from across the plot area at the time of trial set-up and then again one-year later, allowed for estimation of the level of dewatering.
Fig. 2 summarises the end of trial performance of two of the trialled species, Eucalyptus saligna and Vetiver grass, in dewatering the biosolids. Clearly, both species effected a significant decrease in the water content of the biosolids to 30 - 40% in the 5-10 cm sampled horizon. This is relative to a 55% water content for control sampling in May 2003 and a pre-trial water content of 65% at May 2002. Dewatering was so effective that it was not possible to push the soil corer into biosolids below 10cm depth for all cores sampled in proximity to vegetation.
There was a decrease in water content for control cores, sampled away from areas of vegetation. However, we expect that this water will have recharged due to the rainfall of late winter 2003; the timeline of the trial (May 2002 to May 2003) was particularly dry. Dewatering results indicate that in a single growing season, either Eucalyptus or vetiver grass could be used to dewater the top 15cm of sludge stabilising the material and potentially allowing re-use of the substrate. A 15 cm dewatering scenario is conservative and based on observed results; the plants had not been able to reach their maximum biomass over the trial due to cattle and sheep grazing. The true dewatering depth may be much greater, as modelled using the Phyto-DSS.
Robinson BH, Anderson CWN (2007). Phytoremediation in New Zealand / Australia. In: Phytoremediation Methods and Reviews (Ed. N. Willey). Humana Press, Totowa, NJ. pp 455 - 468.