Along the coast of the UK, the British water utilities have been setting up programmes to protect the beaches and simultaneously investing in disinfection technology to comply with EC bathing water standards.
There are a range of different disinfection methods available such as chlorination, filtration and ozonation. The irradiation of wastewater with UV light as a physical method provides numerous advantages:
- it is easy to install and maintain;
- it provides effective and reliable control;
- it is environmentally friendly;
- there is no formation of potentially dangerous DBPs;
- small footprints can be achieved;
- and there are no safety risks with chemical handling.
UV light is part of the electromagnetic spectrum situated between x-rays and visible light (200-400nm). The suitability of UV for disinfection is the result of its photochemical reaction with the cell’s deoxyribonucleic acid (DNA). The DNA’s absorption maximum is close to 254nm within the UV-C region (200-280nm).Throughout several configuration changes, the microorganism loses its ability to reproduce or its ability to synthesize essential protein product. The degree of microorganism reduction is a function of the applied UV dose.
The UV dose itself is independent of wastewater quality. A >3 log or >99.9% inactivation of faecal coliforms will require a UV dose of approximately 25-30mJ/cm2 whether in primary or tertiary effluent. The amount of UV energy necessary to apply this dose is dependent on the wastewater characteristics.
UV lamps
The energy required is generated by UV lamps. The Wedeco Spektrotherm lamp, which is integrated in Wedeco TAK systems for wastewater disinfection, combines the advantages of conventional lamp technologies.
Compared with standard low pressure lamps, the Spektrotherm lamp operates at the same efficiency level but generates three to four times more UV energy. Wedeco is therefore able to offer a considerably lower number of lamps which reduces civil, installation and operation costs.
At large systems such as Preston, the sheer quantity of lamps required tends to point towards medium pressure (MP) lamps. MP lamps have a shorter lifetime (usually 3000 hours at full power). This makes lamp replacement far more frequent. Spektrotherm lamps have a guaranteed lamp life of 12,000 running hours.
Wedeco’s portfolio includes MP lamp technology. But for Preston this would mean increased power and more frequent lamp changes far outweighing any perceived advantage of having fewer MP lamps.
The majority of wavelengths generated by MP lamps are wasted energy, not contributing to disinfection performance at all. Hence, the overall energy costs are usually tripled. Besides the increased operational costs, the costs for larger transformers, motor circuit breakers and power supply lines need to be added to the investment cost.
While MP lamps have always been of variable output, low pressure lamps in the past have been either full on or full off. The adjustment to flow and/or water quality variations is done by switching on/off separate banks. For a three bank installation in a single channel, this gives a switchability of 33.3 – 66.6 – 100%.
Wedeco has recently achieved a breakthrough in low pressure lamp/ballast technology by developing a Spektrotherm lamp whose output can be varied from 50-100%. Besides the variable control, a further enhancement of UV-C output has been achieved resulting in even less lamps being required, further reducing civil works and lowering operating costs.
An important feature in state-of-the-art UV equipment for wastewater is the provision of automatic wiping systems. Wedeco’s automatic wiping system uses PTFE wiper rings which clean organic and inorganic deposits from the protective quartz tubes continuously without disrupting disinfection performance. Standby banks, usually subject to heavy biological fouling, will equally be kept clean. The wiper system runs without chemicals.
Preston’s choice
For Preston, NWW required disinfection with 35mJ/cm2 for a flow of 7853m3/hr with a UV transmission of 30%.
Preston is claimed to be the largest UV system operating world-wide using variable output low pressure high intensity lamps. Manufacture and installation were on schedule and the UV system was fully commissioned by the start of the 1999 bathing season.