Water Journal : Water Journal April 2015
water APRIL 2015 44 Feature article T he sewage that cities and towns have to deal with daily is a source of energy and nutrients that cannot be ignored. Innovative technology that reduces biosolids volume and increases resource recovery is well on its way to commercialisation in New Zealand. People poop. Lots of people produce lots of poop. Every day, Australia has to dispose of some 3,600 tonnes of biosolids produced by treating wastewater and sewage; New Zealand, with its smaller population, only has to cope with 1,100 tonnes. Australia is fortunate to have a farming industry that is willing to accept biosolids, with about two-thirds being recycled to agriculture and other uses on the land, and nearly a quarter stockpiled or sent to landfill. In contrast, the majority of municipal biosolids in New Zealand are sent to landfill, with only 30 per cent reused on the land (see www.biosolids.com.au). Agricultural reuse or land rehabilitation attempts to recover the total nutrient value of biosolids, but it comes with a cost. For example, between 30 and 90 per cent of the total cost of treatment and beneficial reuse of biosolids occurs in the disposal phase. Transport is the largest component of this cost. Typical transport distances in Australia are 200–300 km from the point of generation (DSEWPaC, 2012), often requiring heavy vehicle movement through urban areas. The need for beTTer biosolids managemenT The centenary of the commissioning of the world’s first activated sludge plant will be in 2016. Ninety-nine years ago the new technology delivered impressive improvements in public health and environmental protection. Wastewater and sludge treatment has continued to evolve, but there is still an emphasis on treatment. Opportunities exist for moving beyond wastewater treatment functions like basic sanitation to resource recovery and more. The biosolids from the wastewater treatment are an energy- and carbon-rich resource that contains recoverable nitrogen and phosphorus. Extending value recovery further, biosolids could be used to source materials in the production of chemicals, bioplastics, biofuels and the recovery of trace metals (Figure 1). Bioplastics such as polyhydroxyalkanoates (PHA) have already been synthesised and extracted from treated wastewater-derived biosolids (Chua et al., 2003). Technologies for biosolids management currently in use vary in their costs, their effectiveness at reducing solids volume and biohazard, and in the extent of value recovery. Energy is often recovered through incineration or by using the biogas generated by anaerobic digestion. Incineration reduces volume but is expensive and has a negative public perception. The high moisture content of biosolids can make it challenging to achieve self-sustaining combustion in an incinerator, and supplementary fuel is often required. Anaerobic digestion is a conventional and mature technology. It is relatively inexpensive but only moderately efficient in reducing hazards and biosolids volume; value recovery is also moderate. The end product (digestate) is subject to the challenges discussed above regarding land or landfill disposal. One alternative processing technology is wet oxidation. This process utilises wet combustion through the addition of the biosolids along with an oxidant (usually oxygen) into a vessel operated at elevated temperatures (180–374°C) and pressures (30–90 bar). Under these sub-critical conditions, carbon and nitrogen can be converted and retained as acetic acid and ammonia. new Technology deVeloPed An innovative process to extract value from sewage sludge has been developed at Scion. The target of the TERAXTM process is maximum recovery of energy, carbon, nitrogen and phosphorus components. The heart of the process is a biological fermentation stage combined with a hydrothermal (sub-critical wet oxidation) second stage. This hybrid configuration provides a synergy whereby the strengths of one process mitigate the weaknesses of the other. An outline of the process is shown in Figure 2. Thickened raw primary and secondary solids are fed to an anaerobic fermenter. The sewage sludge is held at 35–55°C and, over a period of four to six days, it undergoes hydrolysis, acidogenesis and acetogenesis, producing acetic acid and other short-chain organic acids. The residence time is relatively short for an anaerobic process, allowing the reactor volume to be minimised with the flow-on effect of reducing capital costs. Approximately 30% of the suspended solids are dissolved during this stage and the vast majority of the carbon is retained within the solid and liquid phases. TOWARDS RESOURCE RECOVERY Biosolids from wastewater treatment is an energy- and carbon-rich resource that in many cases is going completely to waste. John Andrews and Daniel Gapes from Scion discuss the need for better biosolids management and a new way to ‘make poop pay’. Figure 1. Increasing levels of disruption moves biosolids treatment from basic sanitation towards resource recovery (redrawn from a CH2MHILL diagram).
Water Journal February 2015
Water and CSG