Trant builds membrane filter plant trio
Trant Construction is currently building three membrane filter plants in the South. One of the contracts, on the River Itchen in Portsmouth, is worth nearly £13M. Sean Jordan, contracts manager for the company's water engineering arm, reports on the project
Portsmouth Water, an independent water supply company, which serves more than 280,000 customers in and around Portsmouth, commissioned the latest three assignments in Hampshire and West Sussex.
Trant employed design consultant Mott Macdonald and membrane manufacturer Memcor to complement the in-house design team to tackle the three complex sites. Portsmouth Water chose Trant to be the principal contractor at the heart of this project in enhancing water hygiene standards.
More than 100 people are working on three sites: the River Itchen Treatment Works near Southampton and the borehole sites of Soberton, near Winchester, and Fishbourne, near Chichester. Work began in November 2005.
Portsmouth Water is licensed to abstract up to 64Ml of water from the River Itchen every day. And the treatment process to purify the river water into drinking water is the most complex used by the company. The scope of the works for each plant includes design, manufacture, construction, installation, testing and commissioning.
Itchen will be able to handle up to 55Ml/d. Soberton will deal with up to 15Ml/d. Fishbourne has a maximum capacity of 13.6Ml/d. All three sites are designed with a maximum filtrate turbidity of 0.2 NTU and a maximum flux rate of 85l/m2/h.
Portsmouth Water's performance specification requires the plants to achieve varying throughputs/outputs with one membrane unit undergoing maintenance and another membrane unit undergoing a CIP (clearing in place).
Thus the design had to incorporate two spare membrane units, i.e. achieve the required output with two units producing no filtrate. This resulted in incorporating six units into the design, each having a capacity of 14.5Ml/day.
The most onerous design case is a permeate output flow rate of up to 55Ml/day with a maximum sustained filtrate turbidity of 0.2 NTU and occasional spikes of 1.0 NTU (1 hour maximum) with no algal growth in the downstream reservoir and low chlorophyll levels. The maximum flux rate on operational cells, as measured on the external surface of the membrane filters, was 85l/m2/h at 55Ml/day. We elected to base our proposals utilising Memcor's successful CMF-S process in combination with the chlorine-resistant PVDF membranes. The CMF-S system was proven, reliable, secure and met the client's requirements for both a cryptosporidium barrier and turbidity removal.
We adopted a conservative design approach, which maximised the output from each of the strategic treatment works yet minimised capital and operating cost. A conservative design approach was felt necessary for two reasons. Firstly, the feed water was unpredictable, particularly at the River Itchen works. Secondly, while the earlier pilot plant trials proved the performance of the membrane at low turbidity water at each site, the membranes had not been challenged at higher turbidity levels.
And, therefore, the data gathered and available was not complete.
Each of the plants provides a maximum of four log removal of cryptosporidium and clostridium perfringens. The conservative approach adopted means that, at the maximum flows, the design flux is still below the required design flux of 85l/m2/h. In keeping with this approach, if a unit is required for backwash or pressure decay test (PDT), the design is such that there is sufficient buffer in the feed tank to ensure that the maximum flux rate is never exceeded.
At the River Itchen works, feed water enters the membrane plant by gravity downstream of the rapid gravity filters. The scope of works included the installation of 1,500mm-diameter steel pipework laid below ground to divert the flow to the location of the new membrane plant. To take account of backwashes and CIPs, the process requires a balancing volume, within the system. At Fishbourne and Soberton, this was achieved by installing feed water balance tanks. At the River Itchen plant, the balancing volume was achieved by expanding the existing filter water channel by constructing a new in-situ reinforced concrete overflow chamber. At Fishbourne and Soberton, feed water is pumped from boreholes to the new feed water balance tank (about 25m3 volume) within the new membrane plant building.
As the borehole pumps at each of these sites currently pump against a high head to their respective reservoirs, the pumps have had to be replaced to operate against the lower head to the balance tanks. Feed water is drawn through the membrane cells by filtrate pumps. Each cell has a dedicated pump. Filtrate flow passes from the top of each module rack to the filtrate manifold.
The cells' filtrate pump creates suction pressure to give an available trans-membrane pressure (TMP) of up to 85kPa. Flow control is provided via the filtrate flowmeter and filtration pump VSD drive.
The flow setpoint for each cell is calculated from the plant flow setpoint. The flow from each filtrate line is manifolded into a common treated water outlet and directly into the reservoirs or supply distribution network.
Each cell incorporates sufficient freeboard above the racks to accommodate backwash flow interruptions. A vacuum priming system is provided to assist initial cell start-up.
Backwashing is carried out routinely every 60 minutes to remove retained solids from the membrane surface. Backwash water is provided from a washwater holding tank and backwashed through the cell using the cell's respective filtrate pump.
At the end of the backwash, contents of the cell are pumped back to the head of the works at the River Itchen site. At Fishbourne and Soberton, the contents of the backwashed cell are directed to the secondary treatment plant. At all sites this process of washwater recovery minimises water going to waste.
The clean-in-place (CIP) sequence, required to maintain long-term membrane performance, is initiated by a setpoint of TMP resistance or time. The average (design) unit CIP interval is 28 days for each cell at all locations. One cell is cleaned at a time with the others remaining online. The CIPs are carried out using a solution of 300mg/l free chlorine (hypochlorite) and with 0.05% sulphuric acid (PH2). The CIP make-up water is pre-softened via a base exchange softener in order to reduce the risk of hardness precipitation.
The CIP tank has a heater for heating the CIP water so that it enters the membrane cells at temperature of 20-25ºC. The hypochlorite or acid is injected directly into the flow as the CIP pumps recirculate the flow around each cell. After about six hours, the cleaning cycle is completed and the chemical pumped to the neutralisation tank using the CIP pumps. The membranes are then rinsed using feed water and also directed the neutralisation tank.
The scope of works at the three sites comprised the construction of in-situ reinforced concrete tanks and new buildings to house the plants. The building at River Itchen was clad with an industrial composite cladding material.
The building fabric at Soberton and Fishbourne was traditional brick to suit the sites' locations within conservation areas. Further ancillary process, mechanical and electrical installations also within the scope of the works, included:
- Service water and process water supplies
- New chlorination and sulphonation system
- New contact main (Soberton)
- New standby generation plant (Fishbourne)
- Sampling and water quality monitoring
- Permeate monitoring equipment - pH, turbidity and conductivity
- Gas monitoring system
- Safety equipment - emergency showers and eye baths
- Form 4 custom built motor control systems
- Scada system and remote telemetry monitoring
- All site cabling, power and control and instrumentation
- Extensions of and connections to existing pipelines to integrate the new plants
- Drainage, including pumping mains, gravity sewers and interceptor tanks
- Building security systems and general building services