Selecting and Specifying Tanks for Anaerobic Digesters

OK, this is not the most riveting subject to most of us! The reason for blogging about this is that our web site www.anaerobic-digestion.com now has a page devoted to nothing but tanks for anaerobic digesters and biogas plants and includes a new detailed list of UK tank suppliers.

Tanks are available in several different sizes and materials being engineered and designed to be used inside anaerobic digesters and provide long life and leak free containment at digester operating temperatures and pressures.

Tanks that are intended for anaerobic digestion and feed storage are available formed from a wide selection of materials which are selected based primarily on properties of chemical resistance, strength, design life, erection speed and cost. The low initial cost option for the budget digester tank is typically said to be the sectional steel glass or epoxy coated circular steel type tank ( which also should include a zinc coating to the steel, before application of the glass ) seen generally in use as rural/farm digesters.

Digester plant designers do well to note that those that have been coming up with digesters for a long while for sewage sludge infrequently use steel tanks of any type, preferring instead the improved sturdiness and corrosion resistance of a well designed concrete tank.

Concrete also holds the extra benefit of being a good insulator reducing the necessity for insulation in cool climates.

For thermophilic digesters the tank insulation wants should be considered from an initial stage of tank design and selection. Polyethylene ( PE ), a light, chemically-resistant thermoplastic, and GRP ( Glass Reinforced Plastic ) and steel, are the most ordinarily used materials in digester plant subordinate tank applications.

All storage tanks should be supplied with overflow pipes of satisfactory capacity to safely carry off the best quantity of water sure to be discharged by the supply pipe in the event a malfunction of the pump control system fails to stop overfilling.

It is frequently suggested that it is a safe rule to make allowance for the overflow pipe 2 times the diameter or 4 times the sectional area of the supply pipe. Consideration must also be given during design and tank selection to the wants for bunding to reduce the chance of spillage to an acceptable level in the event of tank wall of base failure.

The wants for bunding are stated in the laws administered by the environmental regulator in each location ( Environment Agency in Britain and Wales ). Most tanks have to be installed on a concrete base or reinforced pavers. Never presume for any huge digester tank the ground bearing pressure will be sufficient to support the tank without settlement and damage to the tank base, always make sure that a structural engineer or otherwise suitably qualifies and experience pro assesses the ground bearing pressure at your site before installing the standard tank base. In some examples specially strengthened tank bases or rafts will be required, and at some poor ground bearing sites piling could be needed to support the tank base.

The tanks supplied for digestion plant use are usually needed to be warranted for a minimum of twenty years and the best can be recycled at the end of their useful life. For any guarantee to be defended the purchaser will have to make the proposed use and corrosion traits of the liquid held in any tank clear to the supplier / manufacturer, and go along with any regular inspection or other wants set in the terms of the warranty.

Evidence of a Market Will Become End of "Waste" - EA Says

The Environment Agency (England and Wales) has now published a briefing note which makes it clear that it had changed its previous position that recovered material under the Quality Protocols scheme only ceases to be waste once it's been dispatched to the customer. It has decided that there shouldn't be any distinction made between processed material awaiting despatch and processed material which has already been despatched.

Vincent Brown, Head of Semple Fraser's expert legal team (website www.semplefraser.co.uk) has confirmed, as reported in the CIWM's journal for Waste and Resource Management Professionals published in July, that in law, there never was such a distinction.

The end of waste test needs only that you produce a marketable product that can be utilized in the same way as a normal ( ie, non-waste-derived ) product, with no worse environmental effects. Note the word "can" - not "is".

The legal test needs some evidence of a market to avoid sham production of claimed products that are simply stockpiled ( outside waste controls ) and never meant for consumption, but the method of physical delivery to the buyer wasn't needed.

And this approach is mirrored in Article six of the new Waste Framework Directive ( 2008 / 8 / EC ), which states that "certain mentioned waste shall cease to be waste when it has undergone a recovery operation and complies with express criteria to be developed" as to accord with the conditions, including that "a market or demand exists" for the substance or object.

The EA's new enlightened approach is to be welcomed and must come as very welcome news to many recyclers, as an indication of a more flexible and accommodating perspective to waste-derived products.