Design Of The Advanced Water Treatment Plant

The Advanced Water Treatment Plant (AWTP) was designed following preliminary pre-treatment and Reverse Osmosis (RO) studies. Spiking trials on single RO elements were carried out to compare the rejection of Trent risk compounds (boron, bromide, nitrate and isoproturon) achieved by both High Rejection (HR) and Ultra Low Pressure (ULP) RO membranes. There was some deterioration of rejection achieved by the ULP RO (55 to 40%) for boron removal but all other spiking compounds were removed by >95%. ULP RO elements were therefore selected for the AWTP in view of the energy savings when compared with running HR elements. In pre-treatment pilot trials the performance of self cleaning, fine screen filters was compared with a self cleaning sand filter (Dynasand) prior to UF. Incoming turbidities between 1 and 120 NTU were experienced, with only the Dynasand filter producing filtrate of sufficient quality in all events.

The AWTP was commissioned in May 1998, and comprises a 350 m3/d 'Conventional' stream (based on the flowsheet of Church Wilne WTW) running parallel to a 350 m3/d 'Membrane' stream (Figure 1). Both streams can also make use of ozone and powdered activated carbon (PAC) and the modular nature of the individual processes allows them to be interchanged. Thus, the initial direct comparison will be followed by the optimisation of a process for the River Trent, possibly by combining processes from the two flowsheets.

Conventional Wastewater Plant
Figure 1 Schematic of Advanced Water Treatment Plant

The plant is monitored and to some extent controlled using an 'In-Touch' SCADA package, which also provides historical data trending and can be accessed remotely. Weekly analysis and spiking trials with problem determinands is testing the performance of the various processes, allowing an optimum treatment process for the River Trent water to be developed, based on final water quality and operating costs.

Feed waters to the Advanced Water Treatment Plant (AWTP) are available from four different sources:

- Direct abstraction from the River Trent

- Witches Oak Waters (River Trent from bankside storage)

- Church Wilne Reservoir (impounded Derwent)

- River Derwent (pumped from the Church Wilne WTW reservoir intake line)

The plant can treat a single feed water, or a blend of Trent and Derwent waters, mixed in the inlet break tank. This is important to both Water Companies involved as Severn Trent Water intend initially to treat a blended water from the two rivers and Anglian Water will take the Trent downstream of the Trent/Derwent confluence.

3.1 Conventional Treatment Processes

The conventional treatment stream is based on the Church Wilne WTW flowsheet and comprises clarification, followed by four rapid gravity sand filters (RGF) and four granular activated carbon (GAC) adsorbers.

The cone shaped upflow Clarifier is designed to process 15m3/h of water from the inlet break tank, and uses ferric sulphate coagulant and a starch based polyelectrolyte as a coagulant aid. A height adjustable sludge cone collects the floe blanket and the clarified water overflows under gravity to the RGF header tank. Here it is distributed equally to the four 4.4m (height) x lm (diameter) filters, each of which contains sand and anthracite on a gravel support. The RGF are backwashed every 48 hours or on head loss. The filtrate from all four filters enters a common line and flows to a break tank, which is used both to backwash the RGF and feed forward onto the GAC via a relift pump. The four 6m (height) x lm (diameter) GAC columns are fed from a common header tank and designed to process a combined flow of 14m3/h, the depth of Chemviron F400 activated carbon in each adsorber resulting in a designed empty bed contact time of 30 minutes with all four units in use.

3.2 Membrane Treatment Stream

The membrane stream comprises a self cleaning sand filter (Dynasand), an ultrafiltration skid and a reverse osmosis skid

Raw water is pumped to the bottom of the DST-15 Dynasand and filters up through approximately 3m of 1.2-2mm sand, the filtrate flowing over a weir at the top. The sand is cleaned and recycled by means of an air lift pump running from top to bottom through the centre of the Dynasand. Air, injected at the lowest point of the unit, causes a water/sand/dirt mixture to rise to the top of the unit where it emerges in an open bottom vessel. Filtrate flows upwards into this vessel and escapes over a weir to waste, carrying the dirt with it, the sand meanwhile sinks back down on to the top of the sand bed. The Dynasand filtrate overflows to a break tank from which the Ultrafiltration skid draws its feed.

The Norit MT Ultrafiltration plant (containing X-flow Magnum hollow fibre membrane elements) is designed for a net daily production of 350m3, at a flux of approximately 801m' 2h"'. The unit operates in dead-end mode and is taken off line automatically at pre-set time intervals to backwash or chemical clean (using sodium hypochlorite or nitric acid) to maintain a given trans-membrane pressure at >90% recovery. The filter material is polysulphone based with a nominal molecular cut off of 150-250kD and reduces the turbidity and suspended solids in the permeate to below the limits of detection. The permeate quality is therefore consistent, and any breach in the integrity of the unit can be identified by a detection of turbidity, in particle counts or an increase in the particle index. The UF permeate is collected in a 10m3 break tank, which is used to backwash the UF and feed the reverse osmosis skid.

The Reverse Osmosis skid contains fluid systems polyamide, spiral wound, TFC-ULP 4" membranes and operates in either 4:2:1 or 4:3 configuration. The unit is designed to treat 240m3/day at 80-90% recovery, and is dosed with antiscalant to prevent scaling on the membrane surface. Computer software is used to set the RO plant configuration, antiscalant dose and to assess its performance.

3.3 Ancillary Processes

The ozone plant has been used prior to UF and GAC treatments and is capable of introducing up to 6mg/l of ozone into a flow of 15m3/h with 3 minutes contact time.

The Powdered Activated Carbon plant has to date been used prior to clarification, dosing up to 40mg/l of Norit SA Super PAC. Use of PAC prior to UF will be included within the agreed programme of work.

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