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Rainwater harvesting has become the next best thing when it comes to saving our valuable water supplies. Various techniques are available to make the collection of water possible for use within the home or commercial/public buildings. The pay off in terms of cost and water savings is clearly evidenced by those companies providing these techniques. However, when you look carefully at the projects where rainwater harvesting has been introduced it is rarely used beyond that which is necessary for running washing machines, flushing toilets and watering the garden.
With large scale industry now being encouraged to invest in rainwater harvesting the process becomes a completely different prospect.
In real terms, the cost of collecting enough rainwater to be effectual beyond light domestic needs and to make it safe is a huge investment. The question of contamination makes the use of rainwater complicated. Rainwater contamination can arise from several obvious sources: the roof itself carries dust and dirt from the local atmosphere; the rain water will collect atmospheric pollutants whilst falling, hence acid rain (although this is less of a problem today); bird droppings and feathers as well as the occasional carrion all add to the potential problem as well the debris that is left.
All of these are not an issue if the water is to be used on irrigation, but if you wanted to use it for the preparation and cooking of food products or machine/surface wash down, the degree of filtration and treatment required needs to be of superior standard.
In fact, to ascertain the most efficient ways to save water it is better to look first at a whole production line process, including operational procedures. Rainwater harvesting is not for everybody; it can be like fitting the proverbial square peg in a round hole and be difficult to see any returns in investment. However, there can be other ways to implement water saving strategies that will, in part, help to make an impact when it comes to reducing a businesses carbon footprint.
Case study
We were asked by a food processing manufacturer based in North West England to undertake a feasibility study on rainwater harvesting as part of its carbon reduction agenda.
The food manufacturer was using water primarily in the preparation and cooking of vegetable products and the washing and sanitising of the machinery after each production line process.
The water used as part of the manufacturing could not be recovered as it was heavily contaminated with vegetable and binder products, and past experience had shown that to attempt to recycle this water had the potential to spoil food products used in the next production run.
The ability to harvest rain
This concept is sold as the great saving, but in reality has little impact within industry due largely to two issues: the ability to harvest effectively and to store and make safe for use.
It is often overlooked, but rainwater collection is not as easy as it may first appear. A large roof is the best solution for easy collection, but gathering all the water from any roof to a single point is not always that simple.
This particular manufacturer was based in the North West. Taking the average rainfall of this area over the last four years it was possible that the water collected on roof of the plant would give the manufacturer some gain, assuming that collection was possible and sufficient storage was in place. However, rainwater harvesting is dependent on the rain and, at its best, rain is unpredictable. To accommodate fluctuations it would be necessary to plan on a large size tank to cover the dry times as much as possible, by effectively collecting all the rain available at any given time.
Based on latest rain fall figures for Warrington, with an average of 5cm rain fall per month this would give 780m³/annum or 780,000 litres, which was far short of the manufacturers stated target.
The plant, which was fairly substantial in size, was using 6m³ of water an hour for plant washing. Based on calculations of harvested rain, and assuming it was an average year for rain and all rainwater could be collected, it would get only 50% of its washing requirement. On this basis it would therefore only save the manufacturer £700 per year.
Outcome
Because of cross contamination between the different vegetable products, the re-use of process water was not going to be a suitable option. However, in looking at the entire facility and production process, IPS had noticed that the waste water stream from the production line was heavily contaminated with food residue. Because water is charged on discharge based on the Mogden Formula, this takes into account several variables such as volume, pH, solids and temperature.
IPS recommended therefore that a simple reduction of solids in the water residue would have a positive effect on discharge charges. This in turn would reduce the cleaning load, and the cleaning of the drain sections and go some way to reducing the overall carbon footprint.
Conclusion
The harvesting of rainwater can be effective and can offer significant savings, but potential operators must consider all of the implications in order to evaluate the possibilities.
If you have a large collection area, like a factory roof, then this is a positive for the project. However, don’t forget to give consideration on how it can be collected and delivered to a single point for treatment and processing. Most roofs evacuate rainwater through several downpipes located around the building perimeter which makes collection to one point challenging. Adding pumps at various collection points won’t necessarily solve this problem as there will be no reduction in carbon omissions.
You may consider intercepting your groundwater collection as well as your roof discharge. If your groundwater evacuation lends itself to this then also consider what else you may be collecting other than rainwater, for example: petrol; oil; and diesel spillage from car parks; dust; dirt; and other debris from hard standing areas all of which may have adverse effects on water quality.
You must also consider the application of the rainwater. If it is for a specific process, what quality of water will be required for the process, as this will dictate the type of treatment and the capital cost of the project? If you have a large number of staff, can this be used for lower quality applications such as toilet flushing or vehicle wash water? This will minimise the processing costs and maximise the saving.
Consider the storage of water. Rain is at best erratic and therefore you need to consider what capacity you can accommodate on site. What would be the implications if large storage tanks are to be sited outside? Can they be buried underground? Do you need planning approval? How convenient is its location to the collection and to the potential use of the water?
Also consider your current use of water. Can it be re used? Reusing water only once will reduce incoming water bills by 50%, and this is also the same for effluent charges. This is often the most cost effective solution in water use minimisation.
Safety in collecting and using rainwater is also important as the operator takes the full legal responsibility for its quality and safe use, so you would need to undertake a full risk assessment into what the process requires.
