David Hayward, director of process and environmental management consultant Ashact, outlines an effluent treatment plant upgrade at New Taplow Mill.
The raw mill effluent is initially treated by rotary-drum (2mm mesh) fine screening and primary settlement prior to extensive primary effluent recycling and 100 per cent re-use of primary settled sludge. The mill is thus one of the lowest net water users throughout the UK pulp and paper industry. Prior to the upgrade project, excess high strength primary effluent (typically up to 13,000mg/l chemical oxygen demand (COD)), was pumped to a high-purity oxygen (modified diffused-air) activated sludge process prior to discharge of the treated effluent to a public sewer, within consent limits imposed by Thames Water.
Historically, due to a combination of high primary effluent feed temperatures (up to 45oC) and a relatively long hydraulic retention time within the biological plant associated with the high feed strength (which limited the extent of heat washout via the final effluent), cooling of the activated sludge mixed liquor had been required by the addition of fresh cold water. High temperatures can adversely affect microbiological activity, particularly protozoa and rotifer grazing populations which influence activated sludge floc quality and hence final effluent clarification; this can result in an unacceptable reduction in overall plant treatment efficiency.
For the future, with effluent flows and pollution loads from increased production seeming at the start of the project to warrant a significant increase in secondary (biological) treatment capacity, the use of fresh water for cooling was deemed to be unacceptable, as the effluent volumes to sewer would then exceed the consent. Provision for a heat exchanger cooling system was thus required.
Following a long history of involvement by Ashact in wastewater management issues at the mill (and throughout the whole of the St Regis group), investigations into uprating of the treatment plant were undertaken in early 1995. Apart from cooling plant, at that time, provision for increased primary effluent balancing capacity prior to re-use in the mill was considered to be required.
Several options were sized and costed including use of existing activated sludge tanks as primary effluent balancing tanks, coupled with new oxygen activated sludge plant. After weighing up the options, it was decided that this would be the best way forward. It also offered the lowest capital cost of the four alternatives considered.
This option was accepted by St Regis in principle except that a decision was made to limit increases in design flows and pollution loads. Also, as a result of further consideration of sludge disposal implications, it was agreed that a belt thickener and associated thickened sludge storage tank proposed for handling surplus activated sludge (SAS) would be replaced with a filter-belt press in order to minimise sludge volumes.
The existing oxygen injection equipment was to be retained for maintaining the balanced primary effluent in a reasonably fresh (anoxic) condition prior to its re-use in the mill but the proposed scheme allowed for a new, larger capacity oxygen storage tank to be provided to serve the converted existing aeration (primary effluent balancing) tanks and the new biological plant.
The primary effluent flow was to be delivered to and from the modified existing tanks via new pumps prior to re-use in the mill, with excess primary effluent being pumped via an additional set of new pumps to a separate car park site for secondary treatment.
In the light of appreciable in-house improvements in control over peak:average flows and pollution loads brought about by St Regis, the design basis for the effluent plant upgrading scheme was now considered conservative and did not warrant provision of additional primary effluent balancing capacity.
Thus, a reduced-scale scheme was subsequently agreed, based on a 50 per cent increase on the then excess flows/loads requiring secondary treatment (from an average 4m3/h to 6m3/h) with in-built scope for ready expansion up to 11.3m3/h if required in the longer term.
This revised scheme which included high plant material specifications and electrical/instrumentation control requirements set by St Regis was designed and project managed by Ashact and ultimately commissioned in early 1998 at a total cost of £0.9m. It involved:
- retaining the existing nutrient dosing facilities, three series-operated activated sludge mixed-liquor (ML) tanks and subsequent polymer-assisted dissolved-air flotation (DAF) equipment for secondary treatment duties, but with an additional oxygen activated sludge plant upfront in the form of a single new ML1 tank (355m3) with associated mixed-liquor recycle/oxygen transfer (Vitox) pumps; a new 28 tonne liquid-oxygen storage tank was also included;
- cooling of the new ML1 tank mixed liquor via a sidestream spiral-flow heat exchanger, using river water drawn from a mill header tank via variable-speed pumps linked to triple-validation temperature sensors in ML1; and with the resultant warmed cooling water subsequently delivered back to the mill for production and process cooling purposes but incorporating automatic dumping of excess heat, making use of surplus capacity within the mill’s consent for discharging cooling water to the river;
- a common set of recycled/excess primary effluent pumps with control over selection of the excess flow to secondary treatment via a flow controller/flow control valve;
- transfer of mixed liquor from the new ML1 to the existing tanks (ML2, ML3 & ML4) via mixed-liquor transfer pumps;
- transfer of surplus activated sludge (SAS) from the existing DAF (or standby existing final clarifier), to the existing sludge holding tank for subsequent disposal off-site by tanker (i.e. disposal of SAS in liquid form now retained).
An attraction of the uprated treatment system selected is that previously very high operating biomass concentrations (10,000 — 15,000mg/l) have been reduced to 5,000 — 6,000mg/l within the expanded activated sludge tank volume provided. This allows much reduced solids loadings on an hour by hour basis being applied to the final effluent clarification (DAF) process, giving an improved treated effluent quality passing to sewer.
The uprated PLC-controlled scheme includes new primary effluent recycle and primary sludge return pipelines as part of a general re-alignment and tidying up of the pipework arrangements previously prevailing. All monitoring and alarm information from the treatment plant control room is displayed in the main mill control room, with facilities for historical trend displays and events.
To date, the secondary plant has performed well within target requirements at flowrates up to 8m3/h (design 6m3/h) for COD loads comfortably within design, achieving some 93 per cent COD removal and a treated effluent quality well within the 1,500mg/l COD consent limit. Mixed-liquor temperature has been readily controlled to the design target of 30-32oC.