Rainwater Collection for Hotels and Restaurants

Restaurant Water System InstallationWater is second only to air in terms of value to life. Yet, most new construction projects treat this valuable resource as a problem to be managed. We profess a long term commitment to doing the right thing for water management but we still tend toward the cheapest and most expedient stormwater control methods.

Short term payback rules.  We can’t handle drought, but we can’t handle floods either.   We are supposedly running out of clean water.   What’s the payback period on preventing a flooded basement or worse a flooded community?  What is the ROI when we run out of water?  How do those calculations look when water supply and sanitation costs triple in the next 20 years?

Rainwater Collection & Business Viability

These are all questions for any construction project and are also relevant for new and existing hotels and restaurants.  The hospitality industry has its own water management challenges, which we like to call opportunities.  Several other factors contribute to the increasing viability (and popularity) of rainwater harvesting, these are:

  1. Manufactured rainwater collection components of all kinds are becoming cheaper, better, and smarter. Complex controls that once required human intervention can now be automated, made failsafe, and visible through the internet. If we build a rainwater system for a new hotel or restaurant, we can monitor water quality and other important parameters continuously and more reliably than if we use municipal water. Sheer increase in adoption is bringing equipment costs down.
  2. Water prices are rising (more on water rates) . Water departments have changed from charging less per gallon for high consumers, to more with tiered rates (applause). Although this trend is painfully slow (IMHO!), it is having a positive effect on doing the right thing.
  3. Governments are beginning to charge for stormwater management. The system of storm drains in your 50+ year old subdivision is crumbling, and the developer isn’t going to pay for it. You will pay one way or the other. In some enlightened communities, they offer credits for reducing the stormwater load via on-site capture and infiltration.
  4. In many areas, water crises are developing.  The burning platform caused by the specter of running out of water is motivating people to take a look at alternatives.
  5. In some limited cases, there are fines being levied for failure to adopt rainwater for hotels.  This trend may be coming to your area sometime soon.

The hospitality industry is really ideal for using rainwater collection for these reasons. 

  • Roof size is BIG.  Collection from parking lots and structures is even bigger.  That means a lot of rainwater can be collected.
  • The buildings have aspects of residential buildings with high levels of toilet flushing (restaurants and hotels) and lots of laundry (hotels only).
  • There is usually a significant demand for cooling tower make up, usually in shocking amounts to the uninitiated.
  • Good looking landscape is usually important for business; thus, irrigation is needed.

There are some reasons that alternate water management practices are not more popular:

  1. A tendency toward the status quo of the last 100 years hinders adoption.  While rainwater collection was commonplace at one time, the advent of large centralized water treatment caused us to forget about it for quite a while.
  2. Municipal codes often conflict. Plumbing inspectors are wary of bringing what they think might be nonpotable water into such close contact with drinking water. While water departments appreciate the reduction of consumption during droughts, they also want the revenue.  They have a legitimate concern over the loss of revenue, but we have seen water is typically underpriced and subsidized so prices can increase to cover revenue needs while providing a “carrot” type incentive toward adoption of alternate water sources.
  3. Many rainwater harvesting systems, due to faulty design and installation, just do not work as expected. They have more moving parts than one might think –with a lack of a holistic view on the part of the design and construction teams.  Manufacturers want to make standard products, and they often try to force-fit them. This unfortunately has sometimes led to rainwater collection having a less than positive reputation.  It is best to have an expert do the design and installation rather than a firm who has done it just once or heaven forbid is using you as a guinea pig.
  4. The systems that do work are sometimes more expensive to purchase, install, operate, and maintain than they should be. The water supply estimates must be accurate (see #3 above) and experts know how to make highly reliable systems with less cost than neophytes.  Make sure all operating costs and maintenance are included in your projections.

Rainwater System Design Guidelines

Let’s be clear.  There are ways to reliably address all these concerns.  Designing a rainwater system that’s practical, reliable, and sustainable is definitely within reach if you have an experienced system designer and installer. Here are some guidelines.

  • Decide the primary reason you want this system for your hotel or restaurant. As in most things, there are trade-offs in designing a rainwater system.  Our advice is to try to achieve your top goal and make everything else somewhat secondary be it stormwater management, reducing municipal water use, teaching the public about the benefits of rainwater harvesting, maximizing ROI, meeting corporate sustainability goals, or utilizing every drop of rain. For today’s exercise, let’s assume that our primary goal is to reduce our municipal water consumption as much as possible.
  • Evaluate how much water you have available from rain. If this isn’t your first article on rainwater harvesting, you know that the formula is 623 gallons of water for a 1” rainfall on 1,000 square feet of rooftop minus any efficiency losses. The annual historical rainfall in an area is found at http://www.currentresults.com or other web sites.  Even during a drought, quite a bit of water might be available and your actual capture may not go down as much as you might think compared to normal.  Expert designers will use daily rainfall from specific time periods rather than monthly or annual averages to estimate your water supply from rainwater which always overestimates supply.  If they do not use daily rainfall, raise one eyebrow, and be very skeptical.


For example, Washington DC receives about 40” of rain on average annually. Like most East Coast cities, rainfall is rather evenly distributed throughout the year. Yes, there might be a dry month, but sometimes that’s August and sometimes it’s October.  Even though a 40,000 square foot hotel could conceivably capture a million gallons annually, no viable system will be able to capture and use all 1 million gallons.  There will be system losses and tank overflow, but the goal is to to make that loss as small as possible.  Actual water capture might max out at around 700,000 gallons.  With your goals in mind, you can design a system to maximize capture, maximize ROI, or control runoff or whatever you need to do.

  • The next step is to determine how to use this water. Commercial buildings (hotels and restaurants included) are practically limited to non-potable uses for rainwater and greywater. That’s fine, because it’s such a tremendous opportunity with a lot of non-potable uses. The vast majority of water used in the hospitality industry has non-potable requirements. Here are the uses most practical for the hospitality industry:
  1. The most practical end use is cooling tower makeup. Cooling towers are most often the largest consumer of water. As a rule of thumb, cooling towers use about a gallon of water/month for every square foot of conditioned space during the summer. So a hotel with 7 floors or more and a cooling tower will use all available rainwater for cooling tower makeup.  A nice thing about using rainwater for cooling tower make up is that it tends to be easy as a retrofit opportunity.  Treatment requirements should be minimal unless local regulations have not caught up with technology.
  2. Landscape irrigation is second most practical use of rainwater.  A rule of thumb is that landscape needs about an inch a week of water whether it comes from the sky or elsewhere.  Of course, there are many variables that you landscape architect and irrigation professional can tell you about like ET rates, plant type, sun v. shade, etc.  Again, the only treatment should be pre-filtration and other standard rainwater best practices.  additional treatment should not be necessary.
  3. Washing machines are next. As you may know, in house hotel laundries are water hogs in hotels. We estimate 5-10 gallons per room per night, depending on how often the sheets are washed and you can add restaurant linens, etc.  This is a lot of water, but somewhat more treatment required in this application than with cooling tower makeup and irrigation. If you already recycle your laundry water with a greywater system, kudos to you.  Rainwater can be viable makeup water source for your system.
  4. Toilet flushing is next. This is a common use of water in our experience, but perhaps not as cost effective. The main extra cost is that extra piping is required to deliver treated rainwater to each commode. Plus, many toilet flushing treatment systems are required by code to make the water clean enough to drink. We joke about making rainwater safe for people to drink out of the toilet as if the contaminants in a toilet do not come from you know where! We all know that this level of treatment isn’t necessary, but consumers expect toilet water to be clean and odor-free at the start. Many have progressed past this, and only require sediment to be filtered out to deliver clean and odor-free water.
  5. Full potable rainwater harvesting might actually be less expensive and have higher ROI than using rainwater for toilet flushing alone since we don’t have to run separate piping. However, negotiating the compliance regulations for large scale potable water delivery is beyond the scope of this article, to say the least.

Sizing and Selecting The Right Tank

Sizing the tank is a function of your project goals, roof size, and local rain patterns.  Here are some considerations:

  • Steady rainfall throughout the year with relatively mild ups and downs leads to being able to have a smaller tank for any given amount of water to be supplied. A distinct wet and dry season leads to needing a bigger tank.
  • The tank should be sized to have an occasional overflow for optimal water quality. This means losing a little of your precious resource.
  • If your goal is to never run out of water, even in a drought, your tank may need to be larger than you can afford.
  • If maximizing ROI is your goal, then you probably want to empty and fill your tank as many times as possible through the year. This will mean having a lot of overflow and a lot of empty tank conditions.  This sounds counterintuitive but it has to do with relative capital cost.
  • If your goal is to control runoff you may size the tank to catch the first 1” of rain (and then use it fast!) with any overflow going to designed infiltration.
  • The first gallon of tank capacity will supply a lot more water than the last one added. This means that the bigger the tank, the lower the marginal water supply. Be careful not to oversize your tank and blow the budget.  Your rainwater project may become targets of the dreaded value engineers!

A good rainwater system designer will use daily historical rainfall from select years or time periods to model system performance and figure out which tank size best meets the property owner’s needs.  He may use an average rainfall year for projecting ROI or typical performance or may use a drought year to see how rainwater collection averts problems caused by a drought.

In any case, getting the tank size right is critical to project success because the tank is typically the most expensive equipment component.

Water Collection Tank Types

The type of tank, underground or above, and materials, is an important consideration. On an installed cost basis, above-ground tanks are significantly less expensive than below but offer the opportunity to show off the fact you are collecting rainwater.  On the other hand, below ground tanks can use less space and are less conspicuous, something many property owners prefer. Plastic, above-ground tanks are by far most cost-effective if the required capacity is less than 10,000 gallons. Concrete is durable and cost-effective, especially when integrated into the foundation.

Concrete has the added benefit of reducing rain’s natural acidity. Steel tanks are most common for above-ground commercial installations, giving visibility of conservation efforts while minimizing cost.  Freeze protection is not as complicated as some think, with pipe burial, aeration, and recirculation being enough in all but the coldest climates.

General Treatment

Treatment is the next largest expense. It’s a multi-step process in some cases. Most important is the pre-filter, that keeps leaves and larger debris out of the collection tank.  These filters have coarse screens (around a millimeter or less), and are installed in the downspouts or other piping prior to the tank. Most commercial-scale systems use self-cleaning filters which help minimize maintenance and achieve steady capture.

There are many types of pre-filters (think of mousetraps).  The most effective style for capture efficiency and self-cleaning is the “jump” style although vortex and cascade style filters are fairly prevalent as well.  Regardless of which style pre-filter is used, the important message is that all the water that reaches the tank is quite clean. Left alone, the water actually gets cleaner as it sits in the tank with a well-designed system with proper pre-filtration. A biofilm will grow on the walls, and those organisms clean the water naturally.

For our irrigation and cooling tower makeup applications, the water in the tank is clean and safe enough to use without further treatment. It’s almost certainly cleaner than the water in a cooling tower, which will likely have a treatment system already. You might be asked to add a sediment filter to protect any drip irrigation emitters although this is rarely a real necessity and adds largely unneeded cost and complexity, or that might already be a part of the irrigation design. Coordinate with your irrigation system designers to eliminate redundancy.

Laundry Treatment

Treating rainwater for laundry typically requires sediment filtration to around 10 microns and disinfection.  UV is our recommended disinfection method to minimize the impact on clothing that might occur with chlorine or ozone although if well-controlled or if for whites only, these are certainly very viable.  We just prefer adding less chemicals to our clean rainwater.  If rainwater is combined with laundry water recycling (greywater) then the treatment systems may be combined with rainwater used as make up.

Treatment for Toilet Flushing

For toilet flushing applications, sediment filtration to at least 30 microns is required to keep flush valves working correctly. Filtering as small as 5 microns is common, although that may not be totally necessary.  For even more clarification and odor control, carbon filtration to as low as 1 micron can be used to absolutely assure pristine mountain spring quality water for a perfect flush every time.  The most common form of disinfection is UV with chlorine dosing and ozone also in common use.   Please keep in mind that this treatment regimen is to potable standards and that it is typically done to meet outdated local regulations that require potable water for non-potable purposes.  This is due to worries about liability rather than any particular public health risk.


To make a long story short and to keep non-pump engineers awake, size your pump system to match the standard municipal water supply in both pressure and flow.  Also, make it as reliable as possible and do not make it too complicated.  Choose a designer and installer that understand this and are willing to service your system post-startup.  Pumping and control issues are the most common headache with rainwater systems and require good design and proper maintenance to work right.

Back-Up Water

One final component to discuss is back up water supply. No matter where the rainwater system is located, we will want to air-condition the building, flush the toilets, and maybe irrigate the lawn even during drought. So we will usually have a backup connection to municipal (or well) water. For cooling tower and irrigation applications, an air gap connection into the collection tank is unreliable and expensive when they fail. Far more reliable for any application (both indoor and outdoor uses) is to use a pressure valve (PRV) or a 3-way valve each with a code required backflow preventer.


Rainwater collection can be an extremely viable water source for the hospitality industry and is as of yet rather underutilized.  As water management challenges become more recognized both on the water supply as well as the stormwater management side, we see much wider adoption of rainwater collection in hotels and restaurants.