Taking a lead at Farlington

At Portsmouth Water’s Farlington WTW, which dates back to 1909, Black & Veatch Contracting has installed and is now commissioning a new

84Ml/d MF plant, the largest of its type to be built in the UK.

The existing Paterson Candy works, which provides direct rapid gravity filtration and disinfection to normally good-quality spring water, is to be augmented by the installation of a submerged membrane polishing filtration system. This will provide an effective turbidity and Crypto. barrier to more than 280,000 people and many businesses in the Portsmouth and Havant area. A Crypto. risk assessment carried out for all Portsmouth Water’s groundwater sources by British Geological Survey in 1999 concluded water from Havant and Bedhampton Springs is at significant risk of containing the pathogen. Farlington’s supply is derived from the springs, which are believed to be the largest spring source in Europe.

The project, which was awarded to Black & Veatch Contracting after extensive pilot trials to establish

membrane performance,

preliminary design studies

and competitive tendering, uses the Memcor CMF-S

system, similar to that installed by the contractor for Three Valleys Water’s 40ML/d Chertsey WTW.

spring into action

Throughput of the existing works is dictated by the manual selection of remote source feed pumps and also depends on the level in the downstream treated water reservoirs. The works’ variable flow is governed by the manual selection of duty pumps from the six source pumps. Portsmouth Water controls the pumps with the existing telemetry system. Existing control functions to stop and start the source pumps will be retained.

Feed water to the membrane plant will be supplied under gravity from the existing outlet from the rapid gravity sand filters. The micro-filtered water from the CMF-S units will flow forward to the downstream chlorine contact tank. Chlorine and orthophosphoric acid will be dosed after the membrane filtration.

The plant is designed for unattended operation, running for 24 hours per day.

Operators will visit the works to perform routine inspections and maintenance, carry out chemical cleans if necessary, accept chemical deliveries and attend to all enunciated fault conditions identified via the SCADA/telemetry systems.

The membrane system will consist of six CMF-S units, designed to achieve a minimum net output of 80Ml/d with an instantaneous input of up to 85Ml/d, with an influent turbidity of less than 0.4NTU.

The system is also designed to achieve a minimum net output of 70Ml/d with an instantaneous input of up to 74.4Ml/d with an influent

turbidity of less than 1NTU. To achieve sustainable higher outputs of up to 100Ml/d,

an additional cell is installed but not equipped at this stage.

Each CMF-S unit incorporates 396 micro-filtration modules containing PVDF hollow fibre membranes with a pore size of 0.2µm. Feed water passes through the walls of the hollow fibres to the centre of the fibres producing a filtrate virtually free of suspended solids, which accumulate on the outer surface of the membranes.

The CMF-S units are cleaned with a low-pressure, air-assisted backwash, to maintain the filtrate flowrates.

Filtered water is used as the backwash water medium, flowing from the inside of the fibres, through the membrane wall, removing any accumulated suspended solids from the membranes’ outer surface. Dirty wash water is discharged under gravity to the wastewater holding sump and subsequently discharged to the sewer.

The CMF-S units are chemically cleaned in place (CIP), using either sodium hypochlorite or sulphuric acid, to remove biological growth and fouling which cannot be removed by backwashing alone. The sodium hypochlorite cleaning solution is prepared with heated, softened water from the CIP tank. The sulphuric acid cleaning solution is prepared with service water from the backwash tank. There is no re-use of CIP waste solutions.

All CIP waste and rinse water is directed to a neutralisation tank. Once a CIP is complete, the tank’s contents is recirculated by the duty neutralisation pump, before being neutralised with sodium hydroxide or sodium bisulphite for discharge to the existing drainage system via the wastewater holding sump.

The modules within a

CMF-S unit will be subject to a pressure decay test to check their integrity. The test is automatically carried out on an available CMF-S unit. The filtrate side of the module is pressurised with compressed air to a set value, then monitors the rate of pressure decay over a set period.

The overall automation of the microfiltration plant is achieved with the use of a centralised control (hot/standby PLC configuration) and distributed flex input/output (I/O) architecture supported via a distributed high-speed control network. Plant visibility is provided by a dedicated SCADA system. The centralised controller (ICS) will co-ordinate and interface with each CMF-S unit’s independent PLC, local panel views, and all supportive process units and the existing works’ common control PLC. Membrane process and plant status, water quality parameters and alarms are transmitted via telemetry to Portsmouth Water’s Havant operations centre. The ICS and SCADA software has been written by Black & Veatch Contracting’s in-house engineers.

To minimise environmental impact and achieve gravity flow to the plant, all machinery is housed below ground level. The membrane cells and tanks are constructed from reinforced concrete, lined to resist the CIP chemicals.

A single-storey building, fitted with a full width travelling gantry crane, provides protection for operation and maintenance of the works. The site area will be substantially landscaped, including the planting of native tree species and sowing of wild flowers suited to the local chalk downland.

The programme, which started with pilot trials in October 2000, will be complete shortly, when the plant is expected to go into supply