I fear that the steel industry may have a long search for an alloy resistant to accelerated low water corrosion (NCE 4 June). Peter Howsam of Silsoe College pointed out in your letters column a few weeks ago that the problem is identical to biofouling and biocorrosion in water supply wells. The essential features are the same: an aerobic/anaerobic interface; water flow to provide nutrients; orange layer over a black anaerobic layer.
In dewatering applications where drawdown in the well is often down to the pump intake, the biofouling builds up on the pump. Where the groundwater is brackish, pitting and holing of pumps made of stainless steel (Grade 304 S15) can occur in a few months if the black anaerobic layer is not cleaned off regularly. The UPVC well liner and riser pipe are unaffected but I suppose plastic piles are still some way off!
The precise micro-environment is clearly a key factor in the development of a biomass. In dewatering system pipework biofouling tends to build up fastest in areas of maximum flow velocity (at pump intakes and in small diameter pipework), presumably because of the improved supply of nutrients. This appears consistent with the ALWC experience where the out pans of U-shaped piles and the corners of Z-shaped piles are the principal areas affected, since these are likely to be the areas of maximum flow velocity along a pipe wall.
Toby Roberts, WJ Groundwater, 9 Park Road, Bushey, Herts WD2 3EE.
Hit the spot
Your article on page 8 (NCE 4 June) regarding ALWC is spot on.
It is probably easiest to assume that processes related to biofilms occur in all engineered systems which interface with a natural water environment. As you say, the question is why do they cause noticeable problems in some cases and not in others? The answer is important for confidence in both prevention and remediation measures.
However, ALWC is but one example of a microbially influenced process in a particular environment. Civil engineers generally really do need to build up a better awareness and understanding of microbially activity in/on/under/around their engineered systems. Risk does need to be properly quantified, and the cost benefit of prevention/remediation measures established.
Dr Peter Howsam, Water Management Group, Cranfield University, Silsoe, Bedford MK45 4DT.