Magnesium hydroxide used to aid digestion

Rapid stabilisation of acidic waste sludges can be achieved using caustic soda or lime. But magnesium hydroxide is safer to handle and is much less likely to kill the bacteria needed for anaerobic sludge digestion.

The conventional method of lowering the acidity in a failed digester is to add an alkali such as caustic soda or lime to the sludge, and to displace some of the acidic sludge with fresh sludge containing second-stage bacteria to try and bring the process around. But caustic soda is a hazardous material and lime is usually supplied as a powder in bags, which can make handling and delivery to the digester very difficult.

On the basis that neither lime nor caustic soda were ideally suited to pH control in biological systems, Omex has developed what it claims is a non-hazardous alkali treatment product called Magmex. The principal ingredient of Magmex is magnesium hydroxide.

Aimed principally at treating the acidic waste streams and sludges generated by heavy industries such as metal finishing, the product has now been successfully used to aid the digestion of municipal sewage sludge. Omex Environmental’s managing director Ian Farr said: “Sludge pH is a critical factor in the effluent conversion process. Maintaining the pH balance throughout the process takes time and good management. When using caustic soda and lime to repair a failed digester, care must be taken not to overdose as the exact amount must be applied to bring the pH to within acceptable parameters. Get it wrong and the pH can go up to 12, removing all the natural anaerobic bacteria and making the process non-functional. But Magmex can’t overdose, because it will always buffer out at no more than pH 9.”

Yorkshire Water recently sought advice from Omex Environmental to help stabilise the sludge in one of its anaerobic digesters at Woodhouse Mill STW near Sheffield. The pH of the sludge had fallen below five, leading to a serious breakdown in reactor performance. Woodhouse Mill treats waste from the Mosborough, Swallownest and Woodhouse catchments. The plant comprises an inlet screw pumping station, screens, grit removal, circular primary and storm tanks, activated sludge system, and final settlement tanks. The plant discharges into the River Rother.

Plant operator Gary Stephenson said: “Sewage sludge breakdown efficiency relies on different groups of bacteria, all of which operate in conjunction, but within a defined range of parameters, such as pH, temperature, feed rate, nutrients and trace elements. An imbalance of the parameters had resulted in our digester failing.

“Problems with anaerobic digesters are rare, but when they do occur, considerable logistical problems can result. Acidic sludge is hazardous in that it gives off noxious odours with serious environmental implications if removed from the digester having completed only half of the digestion process.” Despite this anaerobic digestion is the preferred process used by water companies because it produces methane as a usable by-product, reduces sludge volumes, and creates a digested sludge that can easily be dried out.

Alternative treatment systems include incineration and chemical treatment, both of which involve high capital investment and operational costs. According to Mr Stephenson: “Anaerobic digestion is a two-stage process in which bacteria digest the pathogens in raw sludge, which is put into heated and sealed tanks, to produce carbon dioxide, methane and water. An efficient system works at around 300C at about pH 7 and takes a minimum of 12 days.”

“The bacteria required to activate the conversion processes in each stage are very different, with the first stage bacteria being much more resilient to toxic shock caused by particles of noxious elements in the sludge. Without second stage bugs, the process stops working and an acidic sludge is left which is basically unusable.”

Omex provides a delivery and process application service which involves fewer safety procedures than lime or caustic soda. In the case of Woodhouse Mill STW, Mr Farr claimed: “Magmex had been fully mixed into the digester within two hours of delivery, with minimal disruption. Within hours the pH had risen to seven, putting the digester back into service on the same day.” He added: “Using magnesium hydroxide to balance the digester rather than caustic soda or lime is by far the safest and most effective method of pH control.”

Marine origin

Magnesium hydroxide, or Mg(OH)2, is often extracted from mineral deposits on land but can also be produced from seawater. The Mg(OH)2 used to make Magmex is derived from seawater. The process consists of burning limestone, to make lime. The lime is added to seawater to precipitate out the Mg(OH)2. Addition of sulphuric acid to the seawater removes carbonate, and then filtration removes silica and any other remaining solids.

Mr Farr said: “Magnesium hydroxide derived from seawater is of a much higher quality than its mined equivalent as it contains less iron and silica, which are often found in the same rock seams.” These impurities can accelerate metal corrosion, increasing maintenance costs and shortening plant life.

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