Bespoke water treatment
Published: 10 September, 2014
PWE spoke to Dave Clark, commercial & engineering project manager at Veolia Water Technologies, about a bespoke water treatment system for Tata Steel.
Tata Steel is one of the world’s largest steel producers, with operations in 26 countries and commercial offices in over 35 countries. The company is the second largest steel producer in Europe and operates two strip mills in South Wales: Port Talbot and Llanwern. Port Talbot works employs 3600 staff and produces some 4.7m tonnes of steel a year, of which about 3m tonnes is rolled on site and the rest is sent to be rolled at Llanwern, which also has strip coating facilities. The Port Talbot works, which was first opened in 1923, has been the subject of a massive investment by Tata Steel, including a new £185m blast furnace and Basic Oxygen Steel (BOS) making plant off-gas heat recovery system.
The BOS plant produces large quantities of hot off-gas which has to be cooled. Off-gas cooling was, in the past, provided by an open circuit system, but the heat was lost to atmosphere. As part of the upgrade project, Tata Steel wanted to recover the heat energy and specified an evaporative cooling system that uses the hot off-gas to generate high pressure steam. This steam drives a turbine to generate about 10MW of electricity - enough to power 20,000 homes. On-site power generation reduces Tata Steel’s electricity and gas costs as well as their carbon footprint. To produce the required level of purity in the high pressure steam, the new system requires up to 75m3/h of high purity water (<0.2µS/cm conductivity and <20µg/l silica). The design, supply and installation of the vital water purification system was entrusted to Veolia Water Technologies.
The main source of water for the Port Talbot site is the adjacent River Afan, but the water treatment plant also had to be capable of treating a second on-site source of lower quality recovered water in an emergency. The River Afan is typical of upland rivers and its quality varies depending on seasonal and weather conditions. The total dissolved solids concentration is low, typically varying between 70mg/l and 150mg/l but it can increase to as much 400mg/l as a result of de-icing salt being washed into the river from roads in winter. Total Organic Carbon (TOC) is high as a result of organic acids leached from moorland vegetation by the streams that feed the river. During summer the water is low in suspended solids but high rainfall washes colloidal clays into the river increasing both turbidity and colloidal (“non-reactive”) silica. This means that quite extensive pre-treatment is needed before dissolved solids can be removed by demineralisation. The combination of high TOC and low turbidity makes the water from the River Afan difficult to treat. An additional challenge is that the on-site backup water source occasionally suffers from oil contamination which again, must be removed prior to demineralisation.
Dave Clark, commercial & engineering project manager at Veolia Water Technologies explained that Veolia designed and constructed a bespoke water treatment system consisting of pre-treatment by coagulation, high rate clarification and filtration followed by demineralisation by reverse osmosis and ion exchange. In addition to the process plant, the turnkey contract included raw and treated water storage tanks, bulk chemical storage and a 500m2 building for the plant and controls, complete with heating, lighting and ventilation. The scope also included a 1km long 150mm stainless steel demineralised water transfer pipe across the steelworks site including lagging and cladding.
Clark says the plant consists of two sections: pre-treatment and demineralisation treating river water. The pre-treatment section combines a number of processes, the first of which is clarification in a compact Actiflo clarifier to reduce suspended solids, organic contaminants and colour. The process is aided by the addition of a coagulant chemical (polyaluminium chloride) and polyelectrolyte with pH adjustment by hydrochloric acid or sodium hydroxide depending on the raw water conditions.
Actiflo is a high rate clarification process in which coagulated water is flocculated together with microsand and polyelectrolyte. Microsand enhances the formation of robust flocs and acts as ballast, significantly increasing their settling velocity. This allows clarifier designs with very short retention times, high rise rates and extremely compact size. Compared to other clarification processes of similar capacity, Actiflo has a footprint up to 50 times smaller, reacts more quickly to changing raw water quality and allows a shorter start-up time of less than 10 minutes.
The clarified water is dosed with sodium hypochlorite prior to storage to prevent bacterial growth and is then filtered using three Duo two stage filters. The first stage filters are multimedia (sand and anthracite) followed by granular activated carbon to further reduce suspended solids and ensure that organics and oil are removed to an acceptable level for demineralisation. Filter backwash water is recovered and returned to the 600m3 raw water storage tank.
The Demineralisation section has two stages: reverse osmosis and ion exchange. The filtered water passes through three Sirion MegaRO reverse osmosis units to reduce dissolved impurities by up to 98% and organic matter by up to 99%. The high pressure pumps use variable speed drives to minimise power consumption. Reject water from the reverse osmosis units is collected and used for backwashing the filters. A mobile cleaning-in-place system is provided for membrane cleaning.
RO Permeate is collected in a permeate storage tank and pumped to three Rapide Strata ion exchange demineralisation units. These each comprise cation exchange, anion exchange and a HiPol cation polisher. The advanced short-cycle regeneration sequence and stratified anion bed allows regeneration in less than 60 minutes plus reduced chemical and regeneration water consumption compared to conventional ion exchange plant. Demineralised water is stored in a 1200m3 Treated Water Tank with online quality monitoring for conductivity, sodium and silica.
The plant was commissioned early in 2013 and has performed consistently over twelve months in which it has seen a full range of seasonal river water quality variation. It has proved cost effective and the in-plant water recycling and low chemical consumption has met Tata Steel’s water footprint and sustainability goals.
For further information please visit: www.veoliawatertechnologies.co.uk