Introduction

Until relatively recently there has been severe underinvestment in water and wastewater treatment infrastructure in China. This has often resulted in widespread water pollution from untreated sewage being discharged in rivers and lakes, with about half the groundwater in the country now polluted to different degrees. By 2002, over 43 billion tons of untreated industrial wastewater and urban sewage was discharged (1).

Water scarcity is also a major issue, particularly in the north of the country. China ranks fourth in the world in terms of water resources (equal with Canada) but, because its population is 100 times greater than Canada’s, it ranks second lowest in the world in terms of per capita water availability. It is estimated that water shortage is effecting 400 out of 661 cities in China, with 110 cities facing severe water shortages, falling short of requirements by 6 billion cubic metres annually (2).
 
These twin factors of water pollution and scarcity not only threaten human health but also jeopardise China’s economic plans. According to the World Resources Institute, water shortages in Chinese cities caused the loss of an estimated $11.2 billion in industrial output in 2001, while the impact of water pollution on human health was approximately $3.9 billion (3).

Opportunities for UV in China

Water and wastewater treatment and re-use in China therefore represent an area of massive potential.

Every five years the Chinese government puts forward a new Five-Year Plan. The 11th Five-Year Plan, from 2006 to 2010, puts water conservation, wastewater treatment and drinking water safety as high priority issues. During these five years the central government is trying to build a ‘water conservation society’, with special emphasis on the northern region where water shortages are most severe. For example, the central government plans to recycle over 60% of water by the end of the Five-Year Plan, as well as increase sewage treatment rates from the current 45.6% to over 60% in all cities, with a target of 70% in the largest cities (4). Disinfection of water and wastewater for re-use will play an important part in these plans.

To achieve these ambitious targets, China will have to rely heavily on foreign investment and capital. The government therefore recently announced its decision to open its water and wastewater utility markets to foreign investors, with the aim of accelerating the commercialisation of the sector. As a result a large number of European and American suppliers are now establishing themselves in China, bringing with them much-needed expertise.

A treatment technology which is gaining increasing prominence in China is UV disinfection. Since the first municipal UV system was installed in 2001, there are now over 100 such systems in use all over China, with the number growing every year (5). UV is already widely used in Europe and North America – and increasingly in other parts of Asia – to remove harmful pathogens from drinking water and wastewater. Recent advances in UV technology are now also allowing it to be used to reduce pesticides and other contaminants from ground water. UV is a clean technology which does not rely on the use of chemicals and leaves no unwanted residues or by-products.

How does UV disinfection work?

UV is the part of the electromagnetic spectrum between visible light and X-rays. The specific portion of the UV spectrum between 200-315 nm has a strong germicidal effect, with peak effectiveness at 265nm. At these wavelengths UV kills microorganisms by penetrating cell membranes and damaging their DNA – making them unable to reproduce and effectively killing them.

A typical UV disinfection system consists of a UV lamp housed in a protective quartz sleeve, which is mounted either within concrete channels or in cylindrical stainless steel chambers. The water or wastewater to be treated enters at one end and passes through the channel or chamber before exiting at the other end.

There are two main types of UV technology, based on the type of UV lamps used: low pressure (LP) and medium pressure (MP). LP lamps have a monochromatic UV output (limited to a single wavelength at 254nm, whereas MP lamps have a polychromatic UV output (between 185-400nm). LP wastewater systems are usually of an open-channel design with the lamps immersed in channels through which the effluent flows. MP systems, on the other hand, are usually of a closed-pipe design, with the lamps enclosed within a treatment chamber.

Low Pressure or Medium Pressure UV – practical considerations

Traditionally, UV wastewater treatment systems have been of the open-channel, LP variety. With rapid advances in MP technology, however, this is now changing, and closed-pipe, MP systems are now taking over.

A major advantage of closed-pipe UV systems is that they are much more compact and take up much less space than their open-channel counterparts. This means the entire treatment plant can be smaller – an important consideration in urban areas where space is at a premium. The MP lamps used in closed-pipe systems also have a much higher UV output than LP lamps, so fewer lamps are required to produce the same output as LP lamps.

In addition, closed-pipe MP systems are easy to clean. It is well-known that the protective quartz sleeves surrounding the UV lamps in all types of UV systems can become fouled with deposits and minerals. These deposits must be removed at regular intervals to ensure maximum UV output. With closed-pipe systems, cleaning the sleeves is simple, as they utilise a mechanical wiper which moves up and down the sleeve, keeping it free from fouling. Open-channel LP systems often cannot use wipers, relying instead on additional chemical cleaning – a costly and time-consuming process.

Independent research has also shown MP lamps cause permanent, irreversible damage to microorganisms, while some microorganisms can repair themselves after being exposed to LP UV (6). This has serious implications for operators of water and wastewater treatment plant and is another reason why more and more of them are switching to closed-pipe, MP UV technology.

Another drawback with LP systems is that amalgam* LP UV lamps (the most common type) are usually powered by electronic ballasts, which are notorious for having high failure rates. Due to the large number of LP lamps, the corresponding large number of electronic ballasts that need to be regularly replaced can prove to be extremely costly for operators.

Many installations experience both low and high water and wastewater temperatures at various times of the year, or even over a 24 hour period. The efficiency of LP lamps is known to fall off significantly and becomes highly unpredictable at very low or very high water temperatures, and can even fall to zero when the incoming water temperature is below 5oC. MP lamps are unaffected at these temperature extremes – they always start up very easily, show no change in UV output, and do not age any quicker than lamps used to treat water at normal temperatures.

Finally, closed-pipe systems are also safer than open-channel systems, as the UV lamps are enclosed within the treatment chamber so there is no danger of accidental exposure to UV by employees.

The importance of minimum UV dose

A major problem UV systems have to cope with, particularly when treating wastewater, is the fluctuation in wastewater quality caused by variations in the levels of suspended solids and organic loads. These fluctuations affect UV transmittance through the liquid and can, if plant designers do not take them into account, result in variable microbial killing rates. It is therefore essential that any UV disinfection system is designed so that the UV lamps always provide the minimum required UV dose at those points in the system where UV intensity may reach its lowest point. This will ensure that all the micro-organisms in the effluent receive sufficient UV fluence at all times and that killing rates will be more predictable. For this reason optimal lamp positioning is very important when designing a UV wastewater treatment plant.

Water and wastewater applications

Closed-pipe UV systems are highly versatile and can be adapted to treat drinking water, industrial process water and secondary or tertiary treated wastewater prior to discharge or for re-use.

A recent Berson installation in South Korea, for example, has been installed to treat wastewater from the southern city of Sacheon. With a capacity of 26,000 m3/day (1,083 m3/hour), it will be used to remove E. coli from secondary treated municipal effluent prior to discharge). South Korea faces similar water pollution and scarcity problems to China. In Korea, surface water provides over 90% of the country’s water needs and, because the quantity of wastewater generated surpasses the facilities available for treating it, excessive dumping of untreated effluent directly into rivers has led to an alarming rise in pollution (7). The government has therefore begun a major project of building more water and wastewater treatment facilities, as well as modernising existing ones.

UV-treated effluent can also be re-used. This is an important and growing application for UV – particularly in water-poor Asian countries such as China, Korea, Taiwan and Singapore. In Singapore, for example, closed-pipe MP UV systems are being used to disinfect tertiary treated industrial effluent, which is then re-used for industrial applications. Singapore is currently heavily dependant on Malaysia for its water supply, so this kind of installation offers a serious alternative to imports.

UV is also ideal for disinfecting drinking water. Because UV kills all known water-borne pathogens, such as bacteria (including Cryptosporidium) and viruses, it offers a viable, low-maintenance alternative to chlorination. As MP UV kills microorganisms permanently, there is no danger of subsequent re-infection. If required, residual chlorine can be added to the water prior to distribution, but the amount required is very low. In some European countries such as Holland, residual chlorine is not used after UV disinfection because the level of microbial reduction is so efficient.

Conclusion

With the massive investment in water and wastewater treatment and re-use currently taking place in China, those responsible for specifying disinfection systems have an enormous array of options to choose from.

Closed-pipe, MP UV systems have many advantages over more ‘traditional’ treatment technologies and should certainly be given serious consideration. UV is widely used around the world and is known to be safe, reliable and efficient. It is a ‘green’ technology that does not require the use of chemicals and produces no undesirable by-products. Closed-pipe systems are also highly compact and take up very little space – an important consideration in densely populated areas. This technology can make a significant contribution to China’s desire to reduce water pollution and conserve its precious water resources.

ends

References:
1. Xinhuanet, June 5, 2003 (www.grandexh.com/J_Manage/Water_News.asp#)
2. UK Trade & Industry, China Water Sector Report, February 2006
3. World Resources Institute (www.wri.org).
4. http://www.c-water.com.cn/shouce/ziwai/8.htm
5. http://en.ndrc.gov.cn/hot/t20060529_71334.htm
6. Applied & Environmental Microbiology, Vol. 68, No. 7, pp 3293-3299
7. Asian Water, Vol. 18, No. 7, pp 9-12.
* All UV lamps contain small amounts of mercury. Some low pressure UV lamps also contain a small amount of another metal (e.g., indium or gallium) which forms an ‘amalgam’, or mixture, with the mercury. This allows the lamps to operate at a higher current, with a correspondingly higher UV output.