Bio-thermic digester technology

6 min read
Craig Shaw, CEO of Advetec.
Craig Shaw, CEO of Advetec.

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New bio-thermic digester technology, which has been developed in the UK and is attracting international interest, is about to change how water utilities, municipal authorities and other industrial processes deal economically with organic waste, sewage sludge and anaerobic digestate. Craig Shaw, CEO of Advetec, a UK specialist in the use of advanced environmental technologies for the treatment of effluent and waste, explains.

For water companies and municipal authorities across the globe, dealing with sewage sludge and organic material in waste water, whether from domestic sources, gulley cleaning, right through to trade effluent, is a mounting issue. Legislation is pushing hard in one direction, but at the same time volumes are increasing, putting those responsible in a challenging spot.

Sewage sludge is perhaps the biggest issue. The habit of spreading this on the land has declined very significantly over recent years partly due to increasing levels of contaminants such as heavy metals, antibiotics and pathogens, and the general efficacy of using this material as a soil conditioner is not good.

Incineration option

Incineration is another option, but this is only likely to make economic sense where incinerators and sewage treatment works can be co-located, as haulage costs have sky rocketed. Even so, the same water content that makes haulage so expensive also makes sewage sludge extremely expensive and inefficient to burn. Composting isn’t the answer either. Although it is developing into a major processing industry and advances now allow some grey waste (domestic rubbish) and even starch-based plastics to be treated, it remains a very slow process; is still primarily concerned with green waste, and above all is quite unable to deal with the volumes.

Anaerobic Digester (AD) technology has been talked up very successfully as a real option and there are a growing number of AD plants in operation, able to deal with a wider range of organics including sludge. However, we should not lose sight of the fact that AD has been developed primarily to create methane as an energy source. It does this successfully, but the process, albeit faster than composting, is still slow and it is inefficient. With residual waste, typically in the region of 65% of what you started with, you are back to incineration or landfill, so there are multiple costs.

The fact is, in most cases, landfill is still the only real option. And of course with rising landfill taxes and haulage costs, it’s expensive. Costs to the consumer will inevitably rise.

Food and drink problems

Food and drink manufacturers face similar problems, particularly the meat processing sector. The requirement to meet strict wastewater discharge levels in the UK, with similar requirements across the EC and in the US, means food manufacturers have few affordable choices for dealing with meat processing waste and surplus production, except Anaerobic Digestion or landfill. Haulage costs again make this expensive. Organic material in domestic dry waste - often referred to as co-mingled waste - despite the best efforts of some local authorities to encourage separation at source in the UK, is still approximately 40%. Again this is going straight to landfill.

With stricter legislation and new limits on landfill, plus pressure groups and shareholders driving corporate social responsibility, better solutions are clearly needed to break the culture of haul and dump. So what’s to be done? In vessel solutions, we believe, will provide the answer. But to date these have been designed in many cases to compost green waste – domestic garden refuse, municipal grass cuttings, leaves, wood chippings and some food waste – with the goal of producing soil conditioners or fertilizer.

Yes, they are far more efficient than open air windrow composting that can take weeks, but the shortfall of this type of machine is its limitation and lack of control. Throughput can still be slow taking several days and incorrect loading of waste material combined with the inability to control the internal environment, or the digestive rate of the bacteria, frequently leads to poor performance and breakdown of the process, requiring the machine to be physically emptied and then eventually restarted. Nor does this process guarantee a certifiable product, resulting in again waste to landfill.

Entire digestion

By contrast, the Advetec solution is designed to digest organic waste almost entirely and this includes sewage sludge and normally un-digestible organic waste, reducing the volume by over 90% in a 48 to 72 hour period. In addition to sewage sludge, waste from fat traps, septic tanks, filter cakes, gully cleaning and trade effluent, together with green waste and by products from food and meat processing can all be handled by the same technology. The resulting discharge is pure water and a small quantity of sterile powdery residue. With a calorific value similar to that of coal, this can be sold on as refuse derived fuel (RDF) or used on site to fuel bio-mass boilers, producing hot water or electricity. Also, by processing the waste at source, haulage costs and landfill are cut dramatically.

The key difference with the Advetec Bio-thermic Digester is the control and motivation of the bacteria within the system and the replenishment of essential micronutrients that maintain the health and activity of the bacteria - a living zoo if you like. This is crucial; if you change the constituents or balance of waste material entering the system, you need to invigorate the bacteria or activity will slow or stop.

All of the parameters, therefore, within the vessel are fully controlled. This includes the material feed in the vessel, the specific bacteria working at different temperature ranges within each chamber, to the nutrient dosage intervals and rates, plus temperature and humidity. Not only is the bacteriological activity enhanced, but the use of thermophilic bacteria allows the machines to run at temperatures in excess of 180° C, far above other in-vessel solutions. This not only speeds up the process, but makes it possible to digest already part treated material from AD plants, making it a perfect complimentary technology, as well as hard carbons, cellulose and lignin-based materials. Telemetry and 24/7 remote computer monitoring ensures that the whole process can be professionally controlled and optimised.

Trialled, teseted and installed

The result of eight years development the Bio-thermic digester technology has been trialled, tested and installed over the last four years in the US, the UK and the Middle East, covering a range of applications. This includes waste recycling and materials recovery facilities, to water companies, schools and colleges and extends to food and drink manufactures. Working solutions also successfully processed liquid and solid waste for the London 2012 Olympics, so with the announcement of an industrial sized unit, international interest is growing, especially in the US market with organisations like Organic Solutions Inc, Publix and Wal-Mart.  

Unique to the company’s approach is its understanding of aerobic bacteria, the bio-stimulant technology used to invigorate and speed up the process, combined with advanced engineering and continuous remote monitoring to ensure an optimal processing environment.

The latest machine is an industrial scale containerised solution capable of processing 33 tonnes of waste at a time. We calculate that on site processing costs will be in single figures - in the region of £2.50 per tonne, with a typical return on investment in under 18 months. Compare this to current UK landfill costs of £80 to £90 per tonne plus the annual operating costs of haulage vehicles, as much as £100,000 in some cases, and this represents very significant savings for operators dealing with liquefied or solid organic waste.

Modular approach 

A modular approach means the technology is completely scalable up or down; different numbers of digestion chambers can be added as well as different in-feed and out-feed systems. Banks of digesters can be linked together to accommodate different operational requirements and processing volumes. Likewise smaller machines are available to suit smaller waste volumes, making it an ideal on site solution for digesting waste in the food and drinks manufacturing sector as well as for organisations dealing with large volumes of food and organic waste including airports, colleges, schools and hospitals.

In-vessel machines for digesting organic waste will never be a plug and play solution as an element of consultancy will be required for optimisation of the specific waste processing requirements. While it may be possible to integrate some systems into existing processing lines, with no additional operator requirements, other applications will require complete systems consisting of hoppers, shredders, augers, storage tanks and piping, designed and tailored to match each customer’s differing needs.

The good news is that with better understanding of the bacteriological process and the correct application of micro nutrients, in-vessel systems can now deliver the solutions the industry needs to cut the habit of haulage and landfill and to ensure safe and clean discharge into the water system.

The digital edition of the magazine is distributed free of charge to readers who meet our qualifying criteria. You can apply to receive your free copy by completing this short registration form.