Rotorua District Council and Crown Research Institute Scion recently opened a biosolid waste processing pilot plant in Rotorua.
The pilot plant uses thermal deconstruction to break sewage sludge from the council’s wastewater treatment plant down into water, heat, re-useable chemicals, and a range of other valuable by-products. During thermal deconstruction the waste is combusted in water at a high temperature in a pressure vessel.
The destination of most sewage sludge is a hole in the ground – landfill. Scion Group manager for sustainable design, Dr Trevor Stuthridge, says the thermal deconstruction process’s best potential outcome is that none of the biowaste ends up at the landfill but is fully recaptured and used elsewhere. The Scion process, named STOP (Scion Thermal Oxidation Process) can reduce the volume of a given stream of biowaste by more than 90 percent through deconstruction and by-product recovery.
The remaining solid material might be used as a phosphorus fertiliser if suitable. If not, it goes to landfill – but even so takes up a fraction of the space and has almost none of the negatives of the traditional landfill solution. “It has the potential to completely change the way we deal with waste,” Stuthridge says.
The basic technology for deconstructing sludge has been available for more than 50 years, but has only been taken up to a limited extent due to its capital and running costs. Scion scientist Dr Daniel Gapes says New Zealand’s landfill space is becoming increasingly limited. In addition, public opinion and the Waste Minimisation Act 2008 have focused stakeholders on finding other disposal solutions for the large percentage of municipal solid biowaste currently being landfilled. This approach is now a worldwide trend.
“It’s getting expensive to send waste to landfill and, besides, people want to see more environmentally friendly methods of disposal. This means alternatives are on the table again,” Gapes says.
The concepts used in the STOP technology arose from a Ministry of Science and Innovation funded program that focused on developing technologies to improve liquid and solid waste management for the pulp and paper and related industries. Scion recognised the enormous potential to direct some of these technologies towards the difficult issue of sewage biosolids. To work on this issue it created a partnership with its local authority, the Rotorua District Council, which was seeking innovative solution options for this problem.
The partnership team carried out extensive laboratory trials of the STOP technology, and considered the trial results promising enough to take the work to the next level – a pilot plant. Rotorua-based consulting engineers Allan Estcourt Limited prepared the technical specifications for the pilot plant and Hamilton’s Longveld Engineering built it.
The developed world has come a long way from simply dumping raw sewage in the nearest convenient spot and hoping Mother Nature will do her work. In cities, modern sanitation authorities spend huge amounts of money to convey sewage from the point of origin to treatment facilities. The objective of a treatment facility is to separate the raw sewage into clean water and solid materials, or sludge, and then return the clean water to the environment via a series of water purification processes. That leaves the problem of what to do with the solid materials.
The raw sewage goes through several stages of sanitation. Aerobic and anaerobic bacteria play a major role in breaking down the organic material in it. They also work in the treatment system to clean up the water.
There are three types of sludge. The first is primary sludge – the incoming solid material that settles out in the treatment plant’s sedimentation tanks early in the treatment process. It consists of about three percent solids and 97 percent water. The second is secondary sludge – bacterial biomass recovered from the biological treatment process later in the treatment process. It consists of about one percent solids and 99 percent water.
The two sludge streams go through several concentration and water removal processes to produce dry sludge cake. The standard way of dealing with dry sludge is either by landfilling, composting, or (rarely) burning it. Sludge cake contains concentrated nutrients and other contaminants that can enter the environment by, for example, leaching into the water table under a landfill. Burning it is a very energy intensive option and the market for compost has become very limited.
Deconstructing sludge through oxidation processes may be a better option, but has proven very challenging technically and economically. Globally, the process has several variants, with the Zimpro method, developed in the 1950s, being the original technology. Newer supercritical water oxidation processes rely on extremely high temperatures and pressures to accelerate this breakdown.
Scion scientists say the key to the STOP system’s success is to control the thermal deconstruction process to yield by-products for downstream uses, rather than just making the waste into ash, water, and carbon dioxide. The STOP technology’s other advantage is that it is amenable to treating very wet wastes that would not be suitable for incineration. It does this in a manner that recovers value while still destroying fractions that might carry disease, cause toxicity, or are not readily biodegradable.
The oxidative conversion of complex organic material in the STOP system generates simple organic molecules, such as acetic acid, that can then be converted to a range of end products, such as biofuels, biodegradable plastics, and a carbon source for the enhancement of denitrification in the wastewater treatment system. The nitrogen and phosphorus in the solid waste are also converted into clean nutrient-rich fractions that can be used for downstream fertiliser manufacture.
Inside the plant
Certain preventative measures have been built into the pilot plant design to address potential issues of corrosion, scaling, and clogging. The STOP methodology will be tested, refined, and adjusted where necessary during the pilot plant’s work to address these issues. Longveld Engineering business development manager Arthur Lelieveld says it has two interesting features to deal with the challenges associated with sludge deconstruction through oxidation.
Firstly, the reactor vessels were built from austenitic-ferritic 2205 moderate duplex stainless steel rather than standard 304 or 316 stainless steel. This will allow the reactors to withstand the high temperatures and pressure and corrosive media in the sludge mix. The duplex microstructure is known to improve the stress corrosion cracking and pitting resistance of stainless steel against chloride attack.
Secondly, the plant uses a mixture of progressive cavity and gear pumps to ensure consistent performance and better maintenance demands.
It took six months to build the pilot plant, and the building process had its challenges. The Longveld team almost literally had to ‘bend over backwards’ to work and move about in the 8.5m long by 2.5m wide by 2.5m high structure. But Lelieveld says the team took it with good humour. “We had to fit a spaghetti of pipelines into a small area. It was very hard to get everything in the right places. It resulted in some very awkward welding positions. But with meticulous fitting and welding we brought everything to a satisfactory conclusion.”
The adjacent treatment plant supplies the air and water the process requires. A newly installed gas-fired oil heater supplies the heat. Intelectric from Hamilton did the electrical and instrumentation work.
Hard on results, soft on Mother Earth
Scion says STOP has several other friendly features. It uses primary and secondary sludge as a feedstock, which means when used on a commercial scale it could potentially eliminate some of the most difficult challenges of the traditional sewage treatment process.
The process uses energy to start the reaction, but has the potential to be self sufficient and even generate excess heat or steam for other uses – depending on the levels and composition of biomass input and oxidative destruction. In addition, methane can be generated at several stages in the system for electricity production.
STOP is very successful in solids destruction, removing more than 90 percent of the biosolids’ dry weight and sanitising all pathogens. The remaining solids are essentially a fine inorganic ash containing less than five percent of the original carbon content. This material is likely to be biologically stable and suitable for landfill if required. It won’t generate the large levels of greenhouse gas or nitrogenous leachates the original material could have produced under typical landfill conditions.
The way forward
Stuthridge says the process’s eventual commercialisation could benefit the rate paying community and environment alike. “We see the potential of Waste 2 Gold to not only solve our waste disposal problems, but to provide a revenue source from the converted waste for the City Council involved.”
Rotorua has approximately 8,500 tonnes of biosolid waste going to landfill every year at a cost of over $900,000. A commercial STOP plant can change the situation drastically. In addition, commercial ethanol is currently used to improve the sewage treatment system’s ability to remove nitrogen. The acetic acid produced through STOP could replace this ethanol, with potential savings of $400,000 per year.
The technology could be readily implemented in other urban centres, and for applications beyond sewage sludge. Other possible customers include industrial organic waste producers, such as the forestry, agriculture, dairy, and meat and fruit processing industries, as well as chemical companies that may want to generate and recover chemical intermediates from the waste stream as replacements for fossil fuel-derived equivalents.
The current phase of the project will allow Scion and Rotorua District Council to design and cost a full-scale plant for the city’s biowaste disposal needs, as well as identify and test opportunities for other users in the municipal and industrial space.
Meanwhile Stuthridge says Scion has received numerous enquiries from both New Zealand and offshore interested parties.
Christl McMillan, Marketing & Communications Manager, Scion email@example.com