Waste Management & Handling Effective Mixing is Crucial to Maximising the Anaerobic Digestion Process


With increasing volumes of waste now being processed to create biogas from the anaerobic digestion process it is becoming increasingly important to ensure that plant availability and working capacities are maintained. Whilst it is acknowledged that the Biogas industry has benefitted from the technological expertise gained from both the digestion of feed crops such as maize and to some extent from the waste water industry, problems are occurring with other types of waste material. For example, the use of household waste material within the anaerobic digestion industry highlights the problems associated with inconsistent waste materials, even when they have been pre-conditioned and blended before entering the digestion process.


Whilst the sludge’s associated with the waste water industry are usually well screened pre-digestion, and energy crops such as maize are uniform before and during the digestion process, it has been proven that heavier, fibrous materials in ‘waste-to-energy’ digestion processes create new problems. In many applications where submersible and paddle type mixers have shown acceptable results where non-accumulating solids are the norm, there are other examples which indicate a direct relationship between poor mixing and gas production.


In addition to the more commonly accepted problems associated with poor mixing (reduced volatile solids reduction caused by less than ideal anaerobic conditions) other factors should also be considered. These problems directly affect the digester performance in that they either reduce working capacity by settlement or strangle gas release by blanketing the surface of the digester. These problems, combined with solids that are prone to reweave or accumulate in clumps, cause specific problems with unsuitable mechanical plant inside the digester.


Failure to suitably condition simultaneously with mixing also creates problems with concentrated material leaving the digester either via a heat exchanger recirculation system, or onward processing, such as dewatering prior to composting. Premature wear of heat exchanger recirculation pumps caused by grit deposition is also common where poor mixing at low level is prevalent.


Furthermore, transfer pumps can be prone to clogging and premature wear and unless suitable mixing systems are employed inside a post digestion storage tank it is likely that other problems could be encountered with settlement, dewatering or onward transfer to land.


Whilst numerous types of mixing systems have been developed and implemented for use in anaerobic digestion process systems, many have been subsequently discarded. System Mix (UK) with their Vaughan Rotamix system, have become an established supplier of digester mixing systems, not only in municipal installations but also in the rapid growth area of Bio Waste Industry.


A recent example is where Rotamix tank mixing systems were retrofitted as replacements for submersible and paddle mixers on two digesters at a major waste to energy plant during 2010. During the preparation for the retrofit a very significant amount of settled solids were removed from the digester tanks which accounted for over 20% of the available working volume, after only 4 years in operation. The installation of a single externally mounted Vaughan Chopper Pump for each digester and an array of nozzles and pipework were installed quickly and easily.


Since being commissioned gas production has increased significantly and maintained its high level. Post digested sludge condition has shown that the benefits of simultaneous conditioning and mixing create a digestate that is more consistent and more acceptable for use in agriculture.


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info@ps-pubs.com Viridor Expands its Recycling Business in the South West


Viridor (UK) has acquired Martock Waste Paper Limited, a leading materials recycling company head-quartered in Somerset, for a cash consideration of £7.4m on a debt free basis. The acquisition is in line with its parent company, Pennon Group’s strategy of expanding its waste management activities, particularly in the recycling sector.


The acquired company comprises a recycling facility and associated collection activities based in Martock, Somerset, between Taunton and Yeovil. It handles around 50,000 tonnes of paper and other recycled material per annum. The company was established in 1976.


Commenting on the acquisition, Colin Drummond, Chief Executive of Viridor said, “The UK’s waste strategies demand ever increasing levels of recycling whilst the world has an increasing demand for raw materials including recyclate. Recycling is an important and fast growing part of Viridor’s business and accounted for more than a quarter of the Company’s profits in the first half of 2010/11. This acquisition has an excellent geographic and business fit with Viridor’s existing operations in the South West and provides additional volumes for our UK and international recyclates trading arm, Viridor Resource Management.”


Graham Warren, Regional Director of Viridor South West, added: “Viridor has again shown its commitment to recycling in the South


West region. The ever-increasing levels of recycling and landfill diversion being demanded by our customers mean that investment in key recycling operations is crucial. I look forward to welcoming our new colleagues from Martock Waste Paper Ltd, and working together to deliver high quality recycling services to customers across the region.”


Richard Bowers, Managing Director of Martock Waste Paper Ltd, said “I am delighted that Viridor has been able to secure the acquisition


of Martock Waste Paper. This is good news for employees, customers, suppliers and all other stakeholders in the business, and will allow the business to develop.”


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New Oil Spill Waste Processing Guide Released


SLR Consulting (UK) has developed a guide to the processing of waste arising from a potentialclean-up of sections of UK shoreline which could be affected by marine oil spills.


The project, for the Maritime and Coastguard Agency, covers all aspects of the planning and development of appropriate strategies to process the oily waste generated in such incidents. It is intended to assist Local Authority Contingency Planners and clean up teams in pre and post incident planning ahead of any such disaster occurring.


Planning Permission Granted for Waste Treatment and Biomass CHP Facility in Harlow


Sterecycle (UK), the waste treatment and renewable power company, has been awarded planning permission to develop a waste recycling and biomass Combined Heat and Power (CHP) facility at Harlow in Essex, England. The development will treat up to 240,000 tonnes per annum (tpa) of household and commercial waste from the local area achieving high levels of recycling and landfill diversion.


Sterecycle’s Development Director, Alistair Shotliff said: "We’re pleased Essex County Council has approved our plans for the site in Harlow.


"There is a pressing need for waste treatment facilities in this area and this has been acknowledged by the planning authority. Sterecycle offers both local authorities and businesses a solution to the rising landfill tax, and an environmentally acceptable way of diverting biodegradable waste from landfill without relying on mass burn incineration. Using technology such as ours, up to 70% of waste can be recovered and reused - either in the form of valuable recycled products or as biomass rich fibre that can be used to generate renewable energy or as a soil conditioner."


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The guide has been separated into four sections, Local Authority Guidance, Pre-incident Planning, Post-incident Planning and Data and Information. The data and information section contains references to all of the UK facilities which are permitted to process oil spill waste, various other possible outlets for the waste to be re used, such as cement manufacturers and power stations and information on a selection of suitable treatment equipment.


The current documents are seen as the first stage in the development of a comprehensive guide and decision-making tool.


SLR’s Head of Process Engineering, Stuart Metcalf, who had overall responsibility for the project said: “The events in the Gulf of Mexico earlier this year demonstrate how important it is to have contingency plans in place that not only cover disaster management but address how to deal with processing the spilled material.


“We are pleased to have been given the opportunity to produce such a nationally important guide and hope to be able to continue our involvement in its future development.”


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www.pollutionsolutions-online.com • February / March 2011


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