Method, Equipment, and Guidelines for Treating Rainwater to Potable Water
Initial Quality of Rainwater
Water in the atmosphere as it falls as rain is slightly acidic, pure water. Rainwater is surface water from the earth that has undergone a natural distillation process that purifies and removes other elements, chemicals, or contaminants. Contamination, or dirtying, of the rainwater occurs during the rainfall process and as it flows over surfaces such as pavement, soil, and the roofs of structures. The extent that pure rainwater becomes dirty will depend on the quality and cleanliness of both the local air and the surfaces the rain falls on.
Due to the purity of rainwater and its characteristic slightly acidic pH, rainwater is considered an effective solvent. These natural properties increase rainwater’s ability to readily pick up microorganisms, elements, chemicals, and sediments as encountered. Microorganisms such as bacteria, parasites, cysts, and viruses can be found in the atmosphere on dust and organic matter suspended within the air, although fairly rare by comparison. The same is true of chemicals often associated with industrial processes, manufacturing, and transportation exhaust gasses. Some of these are contaminants and pollute the rainwater, while others are harmless but can affect taste, color, and smell.
Rainwater pollutants are unwanted, can cause illness, or can affect health over time and must be removed to achieve potable quality water. Which contaminants are present and in what amounts will vary by location and other details that will be outlined below whenever harvesting rainwater. A recommended step in preparing for a potable rain collection system is to have a sample of rainwater tested and analyzed by a certified laboratory. The best time to get a rainwater sample is after the dry season, after a break in rainfall, and/or during regular rainfall events. Getting a rainwater sample during different conditions can provide a good representation of regular water quality that can be expected and the amount of filtration and treatment that will be necessary.
Equipment Design Overview
Cleaning rainwater to reach potable water quality will typically require a greater input in terms of setup costs, system design, components, product evaluation, installation, and on-going maintenance and inspection requirements when compared to harvesting rainwater for non-potable uses such as outdoor irrigation.
An overview of a potable rainwater harvesting system will include: a catchment area, gutters and channeling, pipes and plumbing, pre-treatment devices, storage tank(s), pump(s), multi-stage filtration, post-filter treatment for disinfection, and a pressure tank (optional but recommended).
Collectively, the concept of purifying rainwater to drinkable water involves taking the rain as it lands then channeling it through gutters, through plumbing, through debris filters, into a storage tank, pumped to a sediment filter, through a finer sized filter, then treated for disinfection before being pumped to inside a building for use or stored in a pressure tank for use when needed.
There are several options available for each of the different steps in terms of setup, equipment, or suitable method. Which option is best or needed will depend on personal scenario and preference. Factors such as amount and type of local vegetation, trees next to the structure, amount of animal life and insects capable of dirtying the system, regional human activity that can introduce atmospheric pollutants, annual local rainfall patterns, and the regular water volume use or consumption rate by the home, business, or farm can all play a role in affecting the setup of a potable rainwater harvesting system.
Equipment, Methods, Guidelines and Recommendations
Having rainwater samples tested for quality and contaminants that cannot be seen can provide great insight into what equipment will be needed, purification methods, and whether upgrading the current basic parts of the system should be considered. Below is a breakdown of the different equipment, methods, guidelines and recommendations for a potable rainwater harvesting system.
1. Rainwater Harvesting Basic Components
The basic components of all rainwater harvesting systems include the catchment area, often a structure’s roof, and channeling to plumb rainwater from its starting point to where it will be filtered, stored, and treated.
For a roof, also called the catchment area, the best material to use is sealed, protected metal that will not release any chemicals, elements or debris into the rainwater. Metal is best for potable rain harvesting due to metal’s cleanliness capabilities, longevity, and as it will not contribute pieces or particles to the rainwater which will need to be filtered out. Asphalt shingle roofs are the most common and are acceptable but can contribute material that needs to be filtered out, are more prone to moss, and are not cleaned naturally by rainwater as well as metal. It is important and recommended to use asphalt shingles that will not leach any petroleum-based byproducts over time.
Channeling, also called the conveyance system, refers to gutters, downspouts, and the piping used to control where rainwater flows. For potable rainwater harvesting systems, high quality metal or plastic (vinyl) channeling is recommended for gutters and downspouts. Gutters are recommended to be fitted with a gutter mesh or gutter guard product as this can significantly reduce large debris collection and subsequent filtering requirements. Gutters without a mesh or guard should be cleaned regularly and should be installed properly to completely drain between rain events.
For all plastic plumbing, PVC pipe is recommended. Only use PVC pipe that carries ANSI and NSF 61 standard certifications. NSF 14 certification is also recommended. Plastic products that are NSF 61 certified are approved for drinking water systems and use in handling drinking water.
Copper pipe is not recommended for rainwater harvesting systems. This is due to the slightly acidic properties of rainwater. Over time, rainwater may cause corrosion to copper piping, which can cause pinhole leaks that can lead to water loss, potential property damage, and eventually need to be replaced. If potable rainwater will go to a hot water tank, a mineral-based pH adjustor may need to be added to prevent damage to hot water copper pipes. Another option would be to use cross-linked polyethylene plumbing, also known as PEX pipe for hot water distribution.
For rainwater storage, common options include rain tanks, rain barrels, IBC totes, and underground cisterns, all made from polyethylene plastic, or metal containers made from galvanized steel or stainless steel. For a potable rainwater system, use only ANSI/NSF Standard 61 certified rainwater collection tanks as they are nationally approved for drinking water.
To Summarize Rainwater Harvesting Basic Components
- Metal is the preferred material type for a rainwater catchment surface or roof.
- The conveyance system should be properly installed, maintained, and gutter screens are recommended.
- Any plastic-based component, such as pipes and the collection container should be ANSI/NSF Standard 61 certified products.
- Copper pipe is not recommended for handling rainwater.
2. Pre-Tank Pre-Filtration
Pre-filtration equipment installed in pre-tank locations within a rainwater harvesting system are used to remove larger debris before it can reach the collection container. Large debris includes vegetation material such as sticks, leaves, seeds as well as small animals, insects, and the like that can be washed along with rainfall. Rain harvesting pre-filtration equipment include downspout screens (also known as leaf eaters or rain heads), tank screens, and first flush diverters.
Any debris should be kept from reaching a rainwater storage tank. Post-tank filtration and treatment is a more fine-sized, sensitive system and any larger debris could cause clogs or render the system ineffective. The amount of debris can often be directly contributed to the immediate area where rainwater harvesting is being practiced. If there are only a small number of trees and vegetation in the rain harvesting area, the amount of pre-filtration treatment needed will also likely be small. Also, if gutters with gutter mesh are used, even less pre-filtration may be needed.
The rainwater harvesting accessory known as a first flush diverter, or roof washer, can help improve total water quality before it reaches the storage tank by flushing away some of the first rainfall that is often the dirtiest. This in turn can lighten the workload on post-tank filtration equipment. However, in a properly set up, well-maintained potable rainwater system, a first flush can be optional.
To Summarize Pre-Tank Pre-Filtration
- Pre-tank pre-filtration is used to remove any larger debris before it can reach a rainwater collection container.
- All larger debris should be captured and removed to prevent clogs and/or malfunctions of downstream equipment.
- Pre-filtration accessories include a leaf eater, first flush, and tank screen.
3. Rainwater Filtration
To purify rainwater to drinkable quality, the water must be both filtered and treated. Filtration refers to using dedicated filter equipment to clean rainwater by removing sediment, some minerals, and other materials that are too small for the pre-filtration accessories, such as pollen and some pathogens out of the rainwater.
Cartridge type filters are common filtration equipment that are suitable and recommended for potable rainwater harvesting systems. Cartridge filters are made to be installed after the rain collection tank and after a water pump. A water pump will be necessary to generate the water pressure (PSI) and flow rate (GPM) required for the filtration equipment to effectively operate. Several rainwater treatment options also require a water pump for these reasons.
For potable grade rain harvesting, it is recommended to use at least two filters in sequence. The first filter is recommended to be a 5 micron sized filter, followed by a 3 micron sized filter. The 5 micron size filter should be capable of removing 99% of all particles greater than 5 microns in size. The 3 micron size filter should be capable of removing 99% of all particles greater than 3 microns in size.
Depending on the quality of the rainwater and the amount of sediment, a sediment cartridge filter may also be necessary. When a lot of sediment is present, the sediment can cause fine sized filters to be overworked and become clogged. Cartridge filters suitable for sediment removal are often labeled for sale as sediment filters and usually have pore size ratings of 20 microns or larger.
For potable rainwater harvesting, ANSI/NSF 53 certified filtration equipment is recommended. Filters that meet the ANSI/NSF requirements will remove 99% of all particles, bacteria, cysts, etc. that are the size of Cryptosporidium and larger. These filters will not be able to filter out viruses, certain bacteria, metal ions, and dissolved organic compounds which are often chemicals. These pollutants can cause illness or health effects over time and should be removed through proper treatment of the water.
An activated carbon filter is also recommended to reduce any potential odors, improve taste, and can remove some impurities that cannot be removed by the other filter types.
If rainwater samples are tested by a laboratory and the report indicates harmful heavy metals or chemicals such as lead, arsenic, and asbestos are present, then filtration equipment rated and listed as capable of removing these pollutants should be added. For potable rainwater harvesting, reverse osmosis filtration is a very effective option capable of removing all pollutants including microorganisms and viruses.
To Summarize Rainwater Filtration
- Multistage cartridge filters are a common and recommended filtration system.
- A water pump will be required for adequate pressure and flow rate.
- Filter sizes should step-down in pore size to a minimum 5-microns, 3-microns is best.
- A sediment filter may also be needed for rainwater systems with substantial sediment.
- ANSI/NSF Standard 53 certified filtration equipment is best and recommended.
- Further rainwater treatment will be necessary to ensure complete purification to drinkable quality water.
4. Rainwater Treatment
To be considered potable grade water, rainwater must be treated so it is pure and clean of pollutants that could cause illness or other health issues. Rainwater treatment refers to disinfecting the water to remove microscopic organisms such as bacteria, parasites, cysts, and viruses. These rainwater contaminants are too small for most regular cartridge filter equipment and will need a specific method of disinfection or filtration equipment that is capable of removing them.
There are three common methods for treating and disinfecting rainwater to reach potable grade quality. Treating rainwater should be performed in the system after larger-scale cartridge type filtration has been performed. Rainwater is recommended to be fully filtered to remove larger particles and sediments prior to undergoing disinfection treatment.
Option One: Ultraviolet Lamp
Ultraviolet (UV) Lamp | Using an ultraviolet light treatment unit is an option frequently chosen for disinfecting unsafe water into drinkable water. Options for UV lamp products vary in design between standalone accessories that add onto a water distribution system or as a combination cartridge filter and UV unit that performs both filtration and disinfection to function as part of a whole home filtration system.
UV lamps must continuously operate in order to disinfect. The UV lamp works by exposing rainwater to high-intensity ultraviolet light as the water passes through a chamber. The extent of UV’s effectiveness in disabling bacteria, parasites, and viruses is directly related to the length of time of UV exposure, flow rate of the water, the intensity/strength of the lamp, and the clarity of the water passing through the exposure chamber.
Most UV light disinfection products designed for water treatment systems are rated for a specific water flow rate and pressure. These ratings are not simply manufacturer recommendations, but guidelines based on the design specifications of the product itself as related to the chamber size, UV lamp size, and strength of the UV emission. Always follow the product’s specific recommendation for water flow rate and pressure requirements or else proper disinfection of the water may not occur.
When using ultraviolet lamps to treat rainwater, Class A UV products that are ANSI/NSF 55 certified are the top recommended choice due to their engineering and performance capabilities to purify large volumes of water effectively, consistently, and long term.
Some important points on ultraviolet light treatment: UV lamp products require a constant electricity supply for operation, a water pump capable of supplying the proper flow rate and pressure, and the bulbs will need replaced after a set duration of use.
Option Two: Reverse Osmosis
Reverse Osmosis | The process of reverse osmosis uses high water pressure and a semipermeable membrane that work together to remove everything from water except the water itself. Reverse osmosis is a highly effective method to purify and disinfect rainwater to reach potable grade quality water. Due to the water pressures required for reverse osmosis, a water pump will be necessary.
A downside to reverse osmosis is the pressure and electricity requirements as well as the wastewater that accumulates and must be discarded during the purification process. The wastewater will contain any impurities removed by the reverse osmosis system and some harvested rainwater will also be lost. Additionally, reverse osmosis systems can operate slowly, require a pressure tank, and can be more expensive, especially products made to handle large volumes of water. If these details are acceptable, reverse osmosis filtration equipment is perhaps the best option for rainwater purification.
Option Three: Chemical Treatment
Chemical Treatment | Treating a rainwater storage tank or system with a chemical method of disinfection is the least recommended method of purification. However, it can be very effective and provides the benefit of residual chemicals that continue to treat the water, keeping it clean, and keeping plumbing free of microbiological growth also.
For potable rainwater applications that will provide a whole structure’s entire water needs on a continuous, daily basis, chemical dosing equipment is recommended. Chemical dosing equipment will continuously administer the disinfectant based on water volume and demand. These products require electricity and regular refilling of the disinfecting agent, which is normally sodium hypochlorite, also known as bleach.
For even more information on rainwater treatment options, such as bleach dosing volumes and more, see our post on How to Turn Rainwater into Drinking Water.
To Summarize Rainwater Treatment
- There are three (3) main options for rainwater treatment: ultraviolet (UV) light, reverse osmosis, and chemical treatment.
- UV treatment is the preferred option and can be the easiest to implement and maintain.
- UV systems should be ANSI/NSF 55 certified Class A systems.
- Reverse osmosis can be the most effective purification option but can be the most expensive and with a few other potential drawbacks.
- Chemical treatment is most often done with active chlorine through bleach or sodium hypochlorite.
- Regular chlorine treatment is best done with a scheduled dosing system.
- Chlorine provides residual disinfection and a few other benefits but requires the chemical to be kept on hand, be measured, and be regularly added.
Takeaway | Rainwater Harvesting for Potable Water
Changing normal rainwater to potable water that is safe to drink and use in all other human use activities requires three steps and a proper rain harvesting system to accomplish those three steps. The first step, pre-filtration, is necessary to remove larger debris that is visible to the human eye and is often performed with gutter guards, downspout screens, first flush diverters, and tank screens. The second step, filtration, is necessary to remove particles, sediment, some microorganisms, and some chemicals and is often performed with multi-stage cartridge filters that step-down in size to remove smaller and smaller materials. The third step, treatment, is the disinfection step needed to remove any pathogens and chemicals that can cause illness or impact health and is often performed with a UV lamp, reverse osmosis, or through chemical treatment.
For more information on rainwater harvesting practice, systems, and applications, review our Rain Tanks Series posts. If you are interested in implementing your very own rainwater harvesting system, we provide ANSI/NSF 61 certified rainwater collection tanks as well as the necessary rain harvesting accessories, including water treatment systems. Large volume galvanized steel rainwater tanks are also available. For any help or inquires, contact our friendly and professional support experts.