. . . 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.


E.coli in different guises has been used as an indicator of faecal

contamination in water for over a hundred years. Detection of bacteria from

the wider coliform group (a taxonomic group long abandoned in classical

microbiology) could be regarded as a historical accident related to the

identification methodology for E.coli. Nonetheless, the detection of

coliform bacteria in water still concerns the regulators and water

companies.

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.


© Faversham House Ltd 2022 edie news articles may be copied or forwarded for individual use only. No other reproduction or distribution is permitted without prior written consent.

Action inspires action. Stay ahead of the curve with sustainability and energy newsletters from edie

Subscribe