. . . that damned elusive coliform
Yorkshire Water Services, in common with other water companies, have been attempting to minimise the incidence of low level coliforms in distribution systems with mixed success writes JG O'Neill public health scientist and J Banks process engineer at Yorkshire Water Services.
The biofilm explanation sloughing off of coliforms growing along with other bacteria on the inside of water pipes has become established over the last ten years as the origin of coliforms found in customers tap samples.
Within the UK water industry Thames Water has published extensively its studies on coliforms in the network and from its experimental pipe rig. Furthermore, UKWIR has instigated a wide range of studies on more fundamental aspects of biofilm development and also on shelf life of distributed water, the organisations involved including WRc, CAMR, KIWA and NANCIE.
It might be expected from all this research that we now understand the humble coliform why, when and where we can expect to find it in the water supply and how to control it. Not necessarily so. Undoubtedly some basic principles have been established higher temperatures, low chlorine, high nutrient levels are all accepted as being favourable conditions for detection of coliforms in customers tap samples. Hence control measures can involve producing low nutrient (biologically stable) water which, along with little or no chlorine is an approach associated particularly with the Dutch.
The high chlorine approach, or the even more effective chloramine, is apparent in any visit to the US. The approach to coliforms and chlorine levels in distribution in Yorkshire Water has been pragmatic. Secondary chlorination is used at some service reservoirs and chloramination has been introduced at some treatment works, but there has been no widespread policy of maintaining high residuals in the network.
The typical summer/autumn peaks in coliform counts are apparent, but there also has been differences between years. Over a 20 year period, there has been in Yorkshire and within other UK water companies, a significant improvement in coliform compliance. At least part of this improvement is likely to have resulted from investment in water treatment, which although not installed to reduce nutrient levels can produce dramatic reductions in bioavailable organic material.
In more recent years coliform compliance has fluctuated with no immediately obvious explanation. This again may be due to fluctuations in bioavailable organic material possibly directly or indirectly related to meteorological effects. Thus a drought results in the drying out of the peat moors, from which 40 per cent of Yorkshire's water is derived. When the rain eventually arrives the colour of run-off water is greatly increased. Although this is removed by treatment there appears to be an increased organic content of the treated water. A similar short term nutrient stimulation effect can also be sometimes seen following heavy summer rain. A week or so later, increases in coliforms are detected in the network too long for an explanation involving service reservoir contamination.
The surprising aspect of these and other explanations is not that we find coliforms occasionally, but that they are not found more often. High nutrient, low chlorine water in a nice warm summer does not always produce coliforms, even when detected they are rarely present in repeat samples. Even more rarely can they be predicted at specific locations. More intriguingly is the experience of Thames Water who cut out sections of main in areas giving high levels of detection of coliforms, but only rarely found any indication of the presence of coliforms on the pipe linings. A further example of the perversely elusive nature of coliforms comes from the Thames Water experimental pipe rig; 1300m of buried 100mm MDP through which has been flowing sluggish, high nutrient water with no chlorine for four years. Perfect conditions for coliforms, but none have been detected either in water samples or in actual biofilm growing on inset studs. The experience of the older NANCIE rig in France has been the same. Furthermore, when large numbers of coliforms (sewage effluent) were dosed into these rigs, little or no colonisation of the biofilm took place.
It¹s not that the biofilm isn't there. In Yorkshire we have established our own experimental system allowing direct sampling of biofilm from live mains in the network. The sampling devices, christened Hedgehogs, are lengths of iron main 1m long 200 mm internal diameter, fitted with removable studs (Figure 1).
Biofilm development on the studs was followed over a year by sampling the studs monthly. The total number of bacteria (as measured using DAPI staining and direct microscopy) peaked at around 108/cm2 after a few weeks but never a single coliform amongst them. However, microscopy has established that the biofilm can be very patchy (Figure 2).
As a separate exercise, in an attempt to track down the conditions for detection of coliforms, the conditions related to routine failures are being examined. For every routine coliform, physical aspects of the site are recorded: pipe material, roughness, age, diameter, and the presence of dead ends. Also, using the LiCWater hydraulic model flow, velocity, mean and maximum age of the water and the likelihood of flow reversals are established. This work is on-going, but by comparison with randomly selected pipes the most significant variable associated with the detection of coliforms is water velocity.
Yorkshire Water has also been involved in studies involving laboratory biofilm generators at the Universities of York and Manchester. These studies have indicted that complex variations in populations of different bacteria in a drinking water biofilm can occur. The bacteria, only rarely previously described, making up these biofilms have also been identified, Variovorax paradoxus being an example. Also, there is very precise evidence that the bacteria found in the water phase have emerged directly from the biofilm. What are the conclusions from all this? Certainly our understanding of the development of water biofilms has improved. Coliforms continue to be elusive, but the large and unpredictable variation in populations of other species of bacteria found in laboratory studies may provide the pattern for coliforms. The precise conditions for supporting a significant population has yet to be identified, but growth sites may be highly localised. From the control angle high chlorine or chloramine levels in the network will reduce but not eliminate the detection of coliform.
If there is a drive in the future to minimise chlorine use, to control
disinfection byproducts, then more attention (than has hitherto been the
case in the UK) may need to be given to the bio-stability of the water as in
the Netherlands. Of course the cheapest (greenest?) solution could be not to
worry too much about coliforms, the worst level failure rates in the
industry are no more than one per cent, rarely failing regulatory
requirements. Certainly these levels do not worry the Dutch who are far more
concerned about disinfection byproducts.