CO2 neutralises basic instincts

Industry is increasingly having to adjust the pH of process water for recycling and waste streams, in response to the continued tightening of environmental legislation. Meanwhile, other companies, with existing neutralisation systems, are finding the need to upgrade them to comply with the new pH consents - which are normally between pH6 and pH10.

In the UK, alkaline waste streams have traditionally been neutralised prior to discharge using strong mineral acids such as sulphuric and hydrochloric acids. However, these acids are potentially very hazardous and require stringent precautions in handling. A particular problem is that accurate process control, maintenance and reliability can be a problem in acid-based systems.

The use of carbon dioxide as an alternative method of pH control, has become increasingly popular in the UK recently. As well as being very cost competitive with sulphuric acid (the cheapest of the mineral acids), CO2 offers a number of additional benefits including improved safety, flexibility, reliability, handling and process control, never mind the cost savings.

BOC Gases, a key supplier of gas-based technologies to the UK water and waste industry, offers a wide range of CO2 pH control systems to meet the differing needs of its customers. These range from small-scale, compact systems operating from a single 34kg cylinder to large-scale bulk systems with the capability of delivering several tons per day of CO2.

Because of the extreme alkalinity of its products, companies in the concrete industry and, in particular, the manufacturers of cement powder and ready-mix concrete, have been particularly affected by the introduction of the latest pH legislation.

CONCRETE PROPOSAL

ARC Northern is one of four regional companies covering England, Wales and part of Scotland, which form the aggregates division of ARC. A key supplier of ready-mixed concrete in the UK, ARC is operating under the trade name Premix.

At ARC Northern's Premix plant at Mold, in North Wales, a BOC Gases Compact3 CO2 pH controller was installed six months ago to tackle the site's process water and effluent. At the dry batch Premix plant, cement and aggregate are conveyed into a truck-mixer vehicle, water is added, and the ingredients are mixed together by the rotation of the drum. The vehicle subsequently delivers the ready-mixed concrete to the customer. The effluent arises from equipment and vehicle washdown, and as a byproduct of concrete production. It has a typical pH range of 12- 13.

This basic water and any site runoff must be reduced to approximately pH9 to comply with local consent levels prior to discharge into the Alyn, a tributary of the River Dee. The system supplied by BOC Gases is an off-the-shelf package which enables those with little or no experience of water treatment to process process water reliably.

The major components of the system include a cylinder of CO2, a gas control panel, pH meter and electrode, distribution pipework and a mixer. The system operates automatically and, at the Mold site, is controlled by a level control switch in the neutralisation tank.

Mike Kean, ARC area manager says: `The arrangement combines runoff and process water into one tank, where it is neutralised by the CO2 dosing system.We actually recycle the water into the product, although when there is high rainfall or lower production, the treated water may be pumped into the river, after the sediment has settled out.'

`Recently we have been recycling 100 per cent of the water and consequently have cut our costs from 28p/m3 of water produced to 8p/m3.'

Once a certain volume of effluent has collected in the tank, a start signal is sent to the control panel activating the pH meter. If the pH of the effluent is above a predetermined set point, the pH electrode in the tank sends a signal back to the control panel. Carbon dioxide gas is injected and mixed into the contents of the tank, reducing the pH.

The Mold installation was carried out by Kentallen Associates and is the seventh BOC Compact pH controller to be fitted for the ARC organisation.