This is part two of a three part series by Sara Earl of Blue Gold Engineering, a water and wastewater firm with a division offering design/permit/build for residential rainwater harvesting, stormwater irrigation and greywater reuse systems.

 

In Part One of this series, I answered some general questions about harvesting rainwater. Here’s a peek into Blue Gold Engineering’s initial system design process to give you a beginner’s understanding of the components of a rainwater harvesting system. Part Three will run Next Thursday, April 28th.

 

Aside from an understanding of your goals, our first consideration is the roof, or catchment area. Surface conditions, slope, direction of flow, total surface area and tree cover all factor in deciding system features and sizing.

 

Do you have gutters and do they have leaf screens? If you don’t already have gutters, you’ll need them to catch water and direct it to the tank. Leaf screens and/or other filters are required to reduce debris in the water and clogs in the system.

 

We’ll assess the physical space you have in your yard for a tank. A tank pad (foundation) can be 17 feet or larger in diameter. If there are space concerns, you may consider more than one tank. There are also, of course, neighborhood height limitations and the top of the tank must be below the lowest gutter point and any overhead power lines or cables. The slope of the site is important if you want to consider a gravity fed system (no pump/electric)—the tank will need to be uphill relative to the area to be irrigated. The location of existing utilities and irrigation infrastructure will also affect the placement of the tank. All this feeds into the decision about the dimensions and volume of your tank.

 

So that’s one way to look at it: how much you can collect. You’ll also need to keep in mind how much area you will want to irrigate, and the specific water requirements of each kind of plantings. If you don’t consider that as well you will end up with too much, or not enough water.

 

From there we decide what size tank, or cistern, makes sense. For the average urban lot, we generally recommend at least a 5,000 gallon tank. There are two reasons. A 5,000 gallon system allows you to maximize both collection and the city rebate. To get the most for your investment, you want to spend about $10,000 on a new system as the city pays up to 50% of costs, up to $5,000. A 2,500 gallon tank can make sense if there’s a very small roof space or a very small area to irrigate. Generally speaking, we recommend a 10,000 gallon tank if you are looking to get your yard through the long, hot summer and a 20,000 gallon tank can keep a family of four “off-the-grid”.

 

If you don’t like metal tanks, keep in mind that a rainwater storage tank can be as simple as a high density polyethylene liner attached to the inside of many different kinds of structures. Tanks can be put underground or shaped with poured concrete. For new construction, storage can be built into a little extra garage space or even into the walls!

 

When it rains, water from the roof flows from gutters into a first flush diverter. The first flush diverter captures a pre-determined amount of water prior to each downspout to help filter debris. Once the first flush diverter is full, the water enters the conveyance: it flows into the downspout, through an underground PVC pipe, through the inlet pipe and into your tank. Alternatively, the conveyance can be an above-ground aqueduct, depending on the terrain and proximity of the catchment area to the tank. A roof washer replaces the first flush diverter for projects with a large catchment area or a lot of downspouts. The roof washer is usually located just before or after the conveyance.

 

If you don’t have the height, space or location to make a gravity-fed system practical, you will need a pump and most urban situations will require a pump. If you plan on installing a system with a pump, you will need to have anRPZ (reduced pressure zone) valve installed. This is the device that prevents the back flow of your rainwater into the city’s potable water pipes. If you have an RPZ, you will need an expansion tank at every hot water heater tank in your home. Water normally backs up into the city line as it heats and cools, but once you install an RPZ valve it can’t. Expansion tanks give the water a place to go when an RPZ valve is present.

 

You’ll need to decide if you want to hand water or install a subsurface drip irrigation system. Your rainwater harvesting system can be connected to an existing irrigation system although existing irrigation lines will need to be painted purple and labeled as containing non-potable water every 5 feet. The addition of sophisticatedcomputerized controls that monitor soil moisture and automate irrigation maximizes conservation and gives you more free time.

 

Our approach equates water quality to it’s highest, best use. Until you are able to use rainwater equivalently to potable water, we think the most productive application is growing food. The cost and energy savings of conserving treated water makes the next best application of rainwater the irrigation of lawn and landscaping. If you are thinking of combining installation of a new rainwater harvesting system with a landscaping overhaul, keep in mind that the city gives rebates if you replace turf with native plants or permeable, non-irrigated hardscaping. 

 

If you’ve always wanted a swimming pool or a creek, pond or fountain to grace your gardens, think rainwater! You can’t run a fountain under the city’s current stage two restriction, but if you’re using rainwater, you are immune. Rainwater can also irrigate green roofs and living walls that have additional energy-saving, environmental and aesthetic benefits.

 

Why you would want to hire an engineer? At Blue Gold Engineering, our “resource balanced design” process delivers the most sustainable and cost-effective solution for your lifestyle, structure and site. Custom design requires an engineer’s understanding of how area, volume, gravity and material qualities relate to slope, pressure, forces and structural integrity. Precise technical calculations will ensure your system is right-sized and effective. For more information about Blue Gold Engineering, please see our web site at www.bluegoldengineering.biz or call us at 512.944.0677.

 

Coming on Thursday, April 28th—Part 3: From Plans to Flow