Maintenance of drainage systems to prevent flooding and water pollution

By David Bleicher
BSRIA Publications Manager

Every building has a drainage system. In fact, most have two – a foul drainage system that takes waste from toilets, showers etc. and a storm/surface water drainage system that takes rainwater from roofs and paved areas. Older buildings may have a combined system, and in some locations the infrastructure buried under the street is a combined sewer – a legacy from the pioneering days of city sewerage systems.

As with maintenance of any building services systems, the first step is to know what you’ve got. Every site should have a drainage plan, showing which drains are located where, what direction they flow in and what they connect to. If there isn’t one, it’s not hard to create one – even though the pipes are buried, there’s plenty of evidence above ground in the form of manholes.

When there is a drainage plan, it’s worth checking how correct and up-to-date it is. Sometimes, the exercise of doing this brings up evidence of mis-connections, such as a new loo discharging into a storm manhole. It’s also worth marking drain covers with the service (F for foul or S for storm) and a direction arrow.

Drainage manhole over showing 'S' arrow to indicate storm drainage and direction of flow.

In foul drainage systems, the biggest headaches are caused by things going down the drain which shouldn’t – like wet wipes, sanitary products and hand towels. So the best form of preventative maintenance is to keep building occupants informed, with polite notices and clearly-marked bins in strategic places. Then there is the fats, oils and greases (FOG) that go down the plughole in catering establishments. If these find their way into the drains and sewers, they’re pretty much guaranteed to solidify and cause blockages – sometimes known as ‘fatbergs’. That’s why there should always be an interceptor in place, also known as a grease trap. This needs maintenance – the generic frequency for cleaning out a grease trap, stated in SFG20 (a common approach to planned preventative maintenance), is monthly. But this will be highly dependent on how the facility is used.

If blockages go unchecked, they may also go unnoticed. That is until sewage starts backing up into the building, or overflowing into storm sewers, which eventually discharge into lakes and rivers. These are delicate ecosystems, and the introduction of detergents and faecal matter can be very harmful to aquatic life and of course humans.

Rain, can pick up contaminants from both the air and the land, so once it has reached a storm/surface water drainage system, it has picked up dirt, oil and chemicals from air pollution, roofs and paved areas. Traditional systems have no means of dealing with this, and also must be sized for occasional extreme storm events, so the pipes are very large and mostly used at a fraction of their capacity. Sustainable drainage systems, or SuDS, attenuate the flow of rainwater to watercourses and emulate the way natural ecosystems treat this water. But they need maintenance. For example, any tree routes that could block a soakaway should be trimmed annually, and green roofs may require weeding on a weekly basis during the growing season.

For more information on the maintenance of drainage systems, please explore the BSRIA Information Service

The hidden menace of corrosion in heating and cooling systems

Written by Reginald Brown, Senior Consultant at BSRIA

Written by Reginald Brown, Senior Consultant at BSRIA

Most buildings services engineers will have come across a heating or cooling system that has not received water treatment and still appears to function perfectly and another that has apparently been treated but experienced serious corrosion related failures. Why should one be vulnerable and the other not? The answer is that most common metals are subject to corrosion but the rate of corrosion and risk of failure depends on a variety of factors including the chemical and microbiological environment, temperature, flow rate and not least the thickness of the metal.

In many respects water is the ideal heat transfer medium for building services. It has a reasonably high specific heat, is liquid over a convenient temperature range and is non-flammable, non-toxic and freely available. The downside is that water is an electrolyte that facilitates corrosion in metallic pipework and components. One might think that the obvious solution is to use plastic pipework but this can actually increase the risk of corrosion of the corrodible components that remain.In a steel pipework system, the dissolved oxygen in the system water will rapidly be used up as it reacts with the large area of corrodible surface but the loss of metal thickness should be insignificant. In a plastic pipework system there are few corrodible components so oxygen concentration will remain higher for longer and the corrodible materials will continue to corrode at a high rate. This means that almost all water based heating and cooling systems should have some form of water treatment to control corrosion, and it may be even more important in plastic pipework systems.

The usual construction programme for large building projects involves installation and pressure testing of pipework followed by pre-commission cleaning and commissioning several months later. During the gap between pressure testing and pre-commission cleaning the system may be both stagnant and still contaminated with manufacturing and construction residues. This is an ideal environment for the development of biofilm and corrosion.

In traditional steel pipe systems (using BS 1387:1985 or BS EN 10255:2004 medium or heavy grade pipe) this is not too much of a problem. The relatively thick pipe (at least 3.2 mm for 1 inch nominal bore and larger) can tolerate the initial corrosion due to the oxygen in the fill water and biofilm development during subsequent stagnation conditions. Provided the pre-commissioning cleaning is carried out effectively, ideally with a biocide wash prior to chemical cleaning, there should be minimal impact on the lifetime of the system.

Thin wall steel pipes and steel panel radiators may not be so fortunate. The thickness of 25 mm nominal bore thin wall carbon steel pipe is only 1.5 mm while a typical steel panel radiator is only 1.3 mm thick. If the initial corrosion was spread uniformly across the metal surface it would not be problem but what tends to happen is that small patches of the surface become anodic relative to their surroundings and are preferentially corroded leading to rapid localised pitting. If dissolved oxygen levels persist or are replenished due air ingress, continuing additions of fresh water or permeation through non-metallic materials then the pitting can progress to perforation. Components that should last 25 years can be perforated in a few months. This is one of the most frequent types of corrosion failure reported to BSRIA and can result in expensive remedial works even before the building is occupied.

Water treatment chemicals work by inhibiting the corrosion process, either by coating the surface of the metal (anodic inhibitors) or otherwise blocking the corrosion reactions (cathodic inhibitors). However, inhibitors are not the solution to poor closed system design or operational deficiencies and certainly won’t work to best effect in a dirty system i.e. one with a high level of suspended solids and/or biological contamination. Also, the system operation must allow the inhibitors and other water treatment chemical to be maintained at an effective concentration and circulated throughout the year.

In summary, the factors necessary to avoid pitting corrosion of steel components in closed systems are:

  1. Minimise the delay between first fill and pre-commission cleaning.
  2. Carry out effective pre-commission cleaning of the pipework system.
  3. Establish, monitor and maintain effective water treatment and water quality as soon as possible in the life of the system.
  4. Circulate water throughout the system on a daily basis to avoid stagnation.
  5. Avoid ingress of oxygen from inadequate pressurisation or excessive fresh water additions.

What happens in the first few weeks of the system can prevent pipe corrosion like this over the next 25 years

What happens in the first few weeks of the system can prevent pipe corrosion like this over the next 25 years

What happens in the first few weeks of the life of the system will influence its fate over the next 25 years. You can’t easily see what is going on inside a pipe but get it wrong and you could be looking at major remedial works in a tenth of that time.

A detailed discussion of corrosion and the use of inhibitors and other chemicals is contained in BSRIA BG50 Water Treatment for Closed Heating and Cooling Systems. Pre-commissioning cleaning is described in BSRIA BG29 Pre-commission cleaning of pipework systems. Guidance on the monitoring of water quality in closed systems is contained in these documents and BS 8552 Sampling and monitoring of water from building services closed systems – Code of practice.

BSRIA also runs a Pre-commission cleaning of pipework systems training course and provides independent failure investigations for all types of building plant and systems including pipweork corrosion.

This article was first published in Modern Building Services.

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