The challenge: Tackling the problem of microorganisms and pesticides
As published in this months WET News….Conventional treatment are effective at removing Cryptosporidium, but there are inherent weaknesses in the technologies. atg’s Barry Hopton reveals all.
“The disinfection of public water supplies is a statutory requirement in England and Wales. Water supplied for domestic or food production purposes must not contain any micro-organism or parasite at a concentration which could constitute a potential danger to human health.” So says the DWI’s Guidance on UV disinfection. The phrase “micro-organism or parasite” includes, of course, Cryptosporidium, which is still a major challenge to the water industry. There are several reasons for this, not least of which is the notorious resistance of the protozoan to the most widely used disinfectant, chlorine. The situation is not helped by the fact that monitoring Cryptosporidium is not easy and, even if it were, the Water Supply (Water Quality) Regulations give little guidance on acceptable levels. Regulation 29 of the 2000 Regulations required that “the average number of cryptosporidium oocysts per 10 litres of water is less than one”. This required a physical barrier. Conventional treatment processes – coagulation, flocculation, clarification and filtration – are effective at removing Cryptosporidium but there are inherent weaknesses in these technologies. Membrane filtration is an effective, if very expensive, barrier and was installed by a number of water companies. Regulation 29 was revoked in the 2007 Regulations and current Regulations require simply that “a water undertaker … shall disinfect the water …”. This places the onus on the water supplier to ensure that the water put into supply is subjected to disinfection which it defines as “a process of water treatment to remove or render harmless to human health every pathogenic micro-organism and pathogenic parasite that would otherwise be present …” This relaxation opened the door to the use of alternative technologies and, for the first time, DWI allowed the use of ultraviolet irradiation as a means of controlling Cryptosporidium and similar pathogenic protozoans. UV had long been proven to inactivate protozoans. A 2004 WHO report summarises research that has demonstrated a 3-log (99.9%) inactivation of Cryptosporidium and Giardia at a UV dose of less than 10mJ/cm2 – a significantly lower dose than is required to inactivate most bacteria. But, given the difficulty of monitoring, how is it possible to be certain that protozoans are inactivated to a safe level?
Three installed 30 lamp UVLW systems featuring the very latest in ultra-high output, low pressure Amalgam UV lamp technology and optimised chamber design
The DWI Guidance on the use of Ultraviolet (UV) irradiation for the Disinfection of Public Water Supplies states that water suppliers “must pre-treat waters … the turbidity of waters to be disinfected should be maintained below 1 NTU. In addition the recommendations … summarised in the Badenoch and Bouchier reports, are still relevant and considered as good operational practice.” It also advises that “Chlorine and chloramine used to provide a residual chemical disinfectant concentration into supply should be dosed after UV processes.”
So UV, at an adequate dose, is an effective primary disinfectant, inactivating bacteria and protozoans, including Cryptosporidium. It produces no dangerous disinfection by-products and involves no hazardous chemicals. The one requirement that UV cannot meet is protecting water in the distribution system but, with the microbiological burden removed, residual chlorination is all that is needed.
The water industry, with good reason, is very conservative and the familiarity of chemical disinfection by chlorine or ozone, easily monitored by redox potential, is somehow reassuring whilst monitoring electromagnetic radiation is largely unfamiliar– in spite of the fact that the pharmaceutical industry has relied on UV disinfection for over a quarter of a century. However more and more water companies are discovering that UV as a primary disinfection strategy reduces the environmental, health and safety risks associated with large scale chlorination or ozonation and gives significant savings in both capital equipment expenditure and operational costs.
Cryptosporidium inactivationachieved with UVLW systems featuring ultra-efficient 800W Amalgam UV lamps designed to provide optimum flow distribution and hydraulic performance
The efficacy of UV disinfection is dependent on the dose of radiation required to handle the microbiological load. The third factor is the UV transmissivity of the water – this is a measure of how much of the applied radiation is absorbed by the water. The lower the transmissivity the higher the applied dose has to be to achieve the required dose. But the critical factor is how the UV dose is delivered and that depends on the design of the UV reactor. UV disinfection has been used for water supply in the United States for over ten years and, rather than re-invent the wheel, the DWI UV Disinfection Guidelines closely references the US EPA guidelines for equipment validation. This requires validation by independent 3rd party bioassay to the EPA protocols.
Validation testing by biodosimetry uses the log inactivation of specific challenge microorganisms passing through a UV reactor, in combination with known UV253.7nm dose-response relationships, to determine a corresponding Reduction Equivalent Dose (RED) and, thereafter, a validated dose for target pathogens. Minimum required REDs derived during reactor validation are expressed in terms of a UV253.7nm equivalent dose. Once the RED for the specific reactor type has been validated then the control system has to maintain it over the full range of works flows and UV transmissivities by monitoring UV intensity in each reactor and automatically adjusting the dose. Not all UV reactors are capable of achieving this.
The first UV systems in the water supply industry were, not surprisingly, installed on good quality borehole sources with high UV transmissivity and fairly constant flows. With increasing experience, and growing confidence, the technology is increasingly replacing traditional chemical systems on surface water sources. The growth has been impressive. The number of atg Evoqua systems delivered under AMP5 has topped fifty, including plants for Affinity Water, South East Water, Southern Water, Sutton & East Surrey Water and Thames Water, with three systems for Portsmouth Water under construction.
So the future looks good for UV disinfection but manufacturers are far from complacent. New developments include high output, ultra-efficient 800 Watt Amalgam UV lamps like atg UV’s 800 Watt UVLX series. These offer a range of benefits including guaranteed lamp life of 16,000 hours, optimized UV reactor design for reduced headloss and a low installation footprint. The ultra-high output of the 800 Watt Amalgam UV lamps reduces the number of lamps, quartz and seals by over 50% by comparison to traditional 330 watt systems, giving significant capital and operating cost savings.