Primary Settlement Tank (PST)

Most treatment plants regard the Primary Settlement Tank as solely a sludge storage tank. WPL have designed their primary settlement tanks with a large surface area specifically to aid settlement, to encourage good crust formation and to achieve extremely reliable BOD (biochemical oxygen demand) removal prior to the settled liquor passing forward to the Biozone for biological treatment. The large and easy to raise lids provide full visibility and access to the PST area to assist with the desludging routine.

Forward Feed system

An air lift system moves settled sewage forward from the Primary Settlement Tank to the Biozone at a pre-set rate. This forward feed mechanism achieves two crucial benefits:

• It maintains an even flow into the Biozone at periods of no- or low flow, thus ensuring the bacteria are constantly fed with nutrient and do not die.

• It lowers the level in the primary tank at times of low flow, creating a buffer capacity ready to accept the next flush of flow.

This combination helps to smooth the flow through the plant, levels out peaks and optimises performance.

Segmented Biozone

The liquor follows a designated path passing in turn through each segment of the Biozone. In the HiPAF, it is impossible for new sewage to mix with partially treated sewage, or even to flush straight through the treatment zone in high flow conditions. In some other plants with a single treatment zone or non-segmented tank such ‘short-circuiting’ can easily occur resulting in poor quality final effluent. The HiPAF’s Segmented Biozone promotes a stable, reliable and efficient process.

The design of the HiPAF’s Segmented Biozone means plants can reliably remove ammoniacal nitrogen to the designated standard. Carbonaceous removal should be separate from nitrification to ensure process integrity. In HiPAFs carbonaceous removal happens in the initial sections with nitrification following naturally in the later sections of the Biozone.

The Segmented Biozone plays another crucial role. In times of low- or even no-flow, the bacteria in the Biozone can die back segment by segment without compromising the process. Each segment naturally revives when the nutrient levels increase again. Thus under-loading does not impede HiPAF plants, whereas under-loading can impede single treatment zone plants. HiPAFs are designed to operate effectively under variable inflow conditions.

Submerged Biogrowth

Unlike RBCs (Rotating Biological Contactors) and Trickling Filter plants, the HiPAF is a Submerged Aerated Filter (SAF) process and is always submerged, therefore, the biogrowth is kept wet, cannot dry out and grows more evenly. The SAF plant process is not susceptible to blockage as with Trickling Filter plants, nor is it susceptible to uneven bio-growth that occurs in RBCs.

Reliable Aeration Process

There are many technical advantages in the HiPAF aeration process. The introduction of air into the Biozone means the bacteria are constantly aerated. This air is also used to scour the excess biomass from the bed of the Biozone ensuring the optimum thickness of biofilm is maintained which is vital for reliable performance.

WPL use random packed ‘Flowcor’ media in the Biozone providing the maximum surface area for bacterial growth with the maximum empty spaces between the media to allow the optimum flow of air generated by coarse bubble diffusion. Research has demonstrated this combination achieves high oxygen transfer efficiency whilst avoiding blockages due to bio-growth.

The baffles in the Biozone are designed to create re-circulation of the settled sewage to reduce potential blockages as well as maintaining the biofilm thickness. The potential for blockage is further reduced since the route through the media bed is upwards, in the same direction as the flow of air. Blockages may be more likely to occur in a down-flow process, especially if not serviced correctly.

Plant Selection Criteria:

Watling Hope and WPL are extremely conscientious when sizing sewage treatment plants since under-sizing is one of the most common causes of ‘plant failure’. Our philosophy is to question assiduously potential clients to assess correctly peak loadings taking into account current and future site populations. Site populations can vary greatly throughout a 24-hour period or on a daily or weekly basis according to peoples’ occupations. Sewage treatment plants should always be sized against ‘peak loadings’.

Easy and Low Maintenance

The HiPAF is designed to be easily serviced and with low running and maintenance costs. For instance, there are no mechanical or electrical components within the plant itself. The open lids give good visibility and access to each section of the plant. The simplified maintenance tasks can be carried out from above which means it is generally more comprehensive than is normal within the industry and means HiPAFs generally achieve a higher level of reliability.

WPL’s Credentials

Many of the ‘water companies’ are in the process of moving to the HiPAF as their preferred plant and technology for use in their own small works. WPL have approved supplier status with South West Water, supply North West Water, Northumbrian Water, Anglian Water and Hyder (Welsh Water), have a framework agreement with Southern Water and have a sole supplier agreement with Yorkshire Water for packaged sewage treatment plants. WPL are the only manufacturer of SAF plants currently accepted by Severn Trent Water.

Overall HiPAF plants provide excellent value and good return on investment over the life of the plant as they are designed and manufactured up to a standard rather than down to a cost.