Although I have worked with rainwater harvesting (at my home in Malta - which is completely self-sufficient in water use through the capture and use of rainwater and greywater recycling and also in Sri Lanka and Kenya), this has limitations when 1) you have low annual rainfall, 2) the annual rainfall is only concentrated within a few months (necessitating large storage capacity) and 3) when you have a huge mismatch between the demand (dependent on theamount of people living in building) and the supply (the amount of rainwater that is falling on and can be collected from the roofs).
Hotels, unfortunately fall in the latter catagory.
Typically most hotels consist of 4, 5, 6, even 10 storey buildings with a limited roof area but which has a high tourist population, and therefore a huge water demand. Even if one were to collect all the rainfall falling on the roof of the hotel, it would only be possible to meet a small fraction of the total water demand.
Sewage is however always available, day in, day out, all year round - as long as you have tourists in the hotel of course. And what's more, if the number of tourists increase (and therefore there's a higher demand for water), there will also be more sewage available for recycling. So there is perfect matching between demand and supply, all year round - independent of occupancy levels.
This is one of the reasons why I have devised the HOTER concept for hotels and large commercial buildings.... Moreover, the HOTER concept does away with the inefficient standard practice of transporting huge amounts of wastewater for treatment/disposal elsewhere; it drastically reduces the energy costs required to produce and transport potable water from the point of production to the point of use; and the net energy requirement for recycling the wastewater is significantly lower than the combined cost of:
conventional potable water production (sometimes necessitating energy-intensive desalination) + potable water transportation (and making good for leaks) + the cost of pumping wastewater to a regional/municipal treatment plant + the pollution cost of leaking sewers + the cost of treating the sewage (and the cost of pumping the treated effluent, if the effluent is to be re-used)
I do not recommend the HOTER concept for individual buildings, because the volumes of wastewater to be treated are simply not enough. The investment in the technology required to ensure that the water is safe without fail will not be justified through the (small) savings in water. However, the HOTER concept makes a lot of sense for small communities e.,g. a village.
The HOTER prototype was tested in Malta, Europe from the period July 2008 to August 2009.
Sewage is treated by a Membrane BioReactor to produce a 2nd class water that can be used for the flushing of toilets, for the topping up of wimming pools, laundry and for landscaping. Surplus treated effluent is then run through a low-energy Reverse Osmosis unit to treat the water to potable standard, which is then re-used for showers, wash-hand basins and baths. In this way, all the wastewater is re-used. The small amounts of sludge generated by the process can be used as a soil conditioner for landscaping.
The investment depends on the capacity of the treatment plant, of course. But as a rule of thumb the HOTER is competitive with conventional methods of water supply/wastewater treatment if the water demand exceeds 75 m3/day.
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by Marco Cremona