When added to the raw water, pumice increases the density of the floc that results from the coagulation process and accelerates sedimentation. Coagulation occurs when the raw water, either surface or bore hole, reacts with the chemical coagulant and the flocs then attach themselves to the pumice particles giving a ballast effect. Settlement occurs under gravity and the clarified water is recovered from the upper part of the clarifier with the sludges being removed from the bottom. This process is suited to potable water, sewage and industrial effluent treatment.

Being of a low bulk density, pumice provides a valid alternative to anthracite in dual media filters. A dual media filter traditionally consists of an upper layer of pumice above a layer of fine sand. Pumice traps most of the large floc particles whilst the finer sand in the lower layer removes the remaining impurities with small particle sizes. The voidage of pumice has an average value of 0.51 which allows for an increased retention of suspended solids in the filter bed and a lower headloss.

Being light in weight, pumice also is able to reduce the operating costs due to the significantly lower backwash water consumption requirements.

Advantages include:

• Savings in energy and water during filter backwash
• Lower bulk density
• Reduced capital cost

3. Aerobic Treatment

Pumice, in addition to its ability to assist clarification, is an essential addition in the Sequencing Batch Reactor (SBR) process where inadequate time has been allowed for phase separation to take place. The addition of pumice just prior to the settlement stage enhances the settlement of the sludge allowing the plant to perform as initially intended.

4. Anaerobic Treatment

Where effluents are highly polluted, the treatment that is finding increasing application is anaerobic treatment or methanisation where the pollutants are treated in the absence of air and methane is generated. Pumice acts as a biomass support in this process, typically employed in the treatment of wastes from breweries, sugar processing, paper manufacturing, diaries etc. The concentration of biomass is significantly increased, growing in ideal conditions on the porous surface of the pumice resulting in reduced retention times and higher methane gas generation.

For anaerobic biomass within methanisation reactors, eg. CPB-concentrated proteins from sugar beet, pumice offers:

• Favourable surface conditions for bacterial growth
• Increased concentration of biomass
• Reduced retention time
• Increased methane generation

5. Sludge Treatment

Sludges derived from the biological treatment of sewage or industrial effluents require further treatment processes to allow their safe disposal. These processes include de-watering where, by the addition of pumice into the mixer prior to the de-watering stage, a superior filter cake is produced. This cake does not clog the cloth, allows greater filter runs with improved filtrate quality and produces a cake that can be used for agricultural or horticultural purposes.

6. Odour Control

In the treatment of sewage and certain industrial effluents, hydrogen sulphide, a highly toxic and malodourous gas is generated. When passed through a bed of pumice, irrigated with final effluent, the hydrogen sulphide is efficiently removed and as the media is not consumed in the process, it offers a long life process solution.

Cheaper than either the conventional peat/heather or mussel shell media, it does not require a carbon source and offers considerable running cost advantages.

Physical Characteristics