Rainwater Harvesting Questions

What can I use rainwater for?

The most common uses of rainwater harvesting include watering your lawn and garden, washing your car and windows; non potable uses such as toilet flushing, and potable uses from drinking and cooking to dish-washing and showering.

Can I store enough water for the whole summer?

Yes. Many west coast residents subsist strictly on harvested rainwater. The most important factor is a large, high quality storage tank. Storage size must be sufficient to get you through the dry summer months. The drier the summers and the higher the summer use – the larger the storage tank required.

How much water can I collect?

How much water do I need?

How big does my storage tank need to be?

Different areas of the west coast have different rainfall amounts and patterns. This changes the important factors (roof area and storage tank size) required to meet your water needs. If you plan on making it through an entire summer you’ll need to know the following:

Number of residents X number of days residing X average daily usage = summer harvest requirement.

Example

2 people spending their weekends at the cottage for June, July, and August, using a conservative 110 Litres per day:

2 residents X 24 days X 110 Litres/day = 5,280 Litres.

This couple would need over 5,000 Litres of water to get them through the summer to satisfy their indoor needs. If their average summer rainfall harvest is only 1900 Litres, as on Salt Spring Island, they would need a minimum 3,100 Litres of storage capacity (5,000 – 1,900 = 3,100 Litres).

How much filtering do I need to do?

The amount of filtering required in your system depends whether your usage is for outside use, toilet flushing or drinking.

There are two basic considerations:

1. SCREENING AND FILTERING THE RAINWATER BEFORE IT ENTERS YOUR STORAGE TANK.

Primary screening can be screens that cover the entire gutter system to take out twigs, leaves and needles from close-by trees before entering the downspouts. There are a number of products on the market that do this job. Keep in mind accessibility for periodic cleaning.

If debris from trees is less of a problem then filter screens (rain heads) can be installed at the top or bottom of the downspouts. Some devices are designed to shed debris (self-cleaning) and others have a collection pocket or canister that can be emptied periodically.

A third method is to install a filter basket that fits inside the 16″ lid opening at the top of most storage tanks. The filter basket can be used in addition to gutter and downspout screens and has the added advantage of accommodating an optional fine cloth filter that will filter out pollen as well. Keep in mind all screens and filters must be easily accessed.

The more screening and filtering you can do before water enters the tank means there will be less sediment buildup in the tank, and less future cleaning.

2. FILTERING THE STORED WATER AS IT LEAVES THE STORAGE TANK.

Further filtering on the outlet from the tank depends on the type of use.

a) If used for outside use only (garden, washing windows or cars) then no additional filtering would be necessary.

b) Water for toilet flushing and laundry needs to be filtered to a higher level to ensure pipes and valves do not clog up or become damaged from particles in the water.

c) Water for drinking or human contact must be further filtered to one half a micron to ensure the proper function of an ultra-violet (UV) disinfection unit to ‘purify’ the water for human consumption or contact. As the water quantity needed for drinking or showering is relatively small, a separate tank or separate connection to the main storage tank would save the cost of unnecessarily disinfecting water that would be going to outside use or toilet flushes.

A professional person familiar with UV disinfection must be consulted for this application.

There are a number of filtering devices on the market, and a rainwater professional can advise what is best suited for your application.