Membrane solution to a “significant risk” Cryptosporidium groundwater source

Collaborative work between Thames Water Utilities and PCI–Leopold outlines how pilot trial work has been taken through to full-scale application of a 9 Ml/d, ultrafiltration plant on a “significant risk” Cryptosporidium groundwater source.

The pilot plant was subjected to a wide range of feed water quality fluctuations which had very little effect on the permeate quality. The single UF treatment step demonstrated complete coliform removal, regardless of the influent counts. Larger sized particles such as protozoan species, Giardia and Cryptosporidium will be expected to show similar size exclusion trends. Particulate solids were significantly reduced by size exclusion and cake formation removal processes. TOC showed no discernible reduction due to the pore size/MWCO of the membrane. The trials demonstrated that the membrane could be run in a stable manner for a period in excess of 4 months at a flux of between 120 and 130 l/m2/h and a cleaning regime of 30-min backwash intervals and 8–12 h CEB intervals. Scaling control by infrequent acid washes are likely to prove beneficial in combating calcium build-up. Some membrane tenders/contracts have resembled “flux wars” between suppliers. Whilst this may result in an initial low capital cost, the associated opex has often been high. As a result, this scheme was designed and let on a “whole life cost” basis, incorporating chemical and power consumption rates, membrane life expectancy, etc. Many of these issues are closely correlated to “sustainable” operating conditions, especially flux rates and normal TMPs. This accounts for an average flux selection of 125 l/m2/h. The higher value of 140 l/m2/h demonstrated TW could “sweat the asset” for short-term periods if it needed to. For this raw water, the value was likely to reach the maximum achievable flux. 7. Installation of a full-scale plant 7.1. Current status The engineering contract to install the fullscale plant and refurbish the works started in April 2000 and is nearing completion. Miller Water in conjunction with Binnie Black and Veatch aided in design. The UltraBar installation will be supplied by PCI–Leopold in a primary pass configuration. The backwash water will not be recovered, and instead will be discharged to the sewer. Water leaving the site will be dosed with hypochlorite across a flow balancing tank for residual control and distribution management Fig. 2. Process flow schematic.