Quick Facts on Rainwater and Rainwater Harvesting.
- Uganda has two wet seasons commonly referred to as bimodal. The rainy season is from March until May and August until November. Light rain season falls in November and December.
- Uganda is endowed with plenty of rainfall. Annual rainfall ranges from about 650mm in the North Eastern region to about 2,200mm over Lake Victoria basin in the Central region.
- Uganda receives 66,000 million cubic meters of water from rainwater annually.
- Rainwater is the purest form of natural water, known for its softness and superior taste. This is because it forms from evaporated vapor, leaving behind impurities. Impurities are picked up and dissolve as it falls (atmospheric pollutants as gases, dust, smoke, microbes, air particles, pollen) or flows over collection surfaces (catchment pollutants as bacteria, bird droppings, chemicals, debris, minerals) like roofs, gutters, land, or rocks.
- Rainwater harvesting is dependent on rainfall patterns, thus not always possible to depend solely on rainwater harvesting as a permanent source of water supply. It is used as an additional water supply source.
- Legislations around rainwater harvesting vary around the world. While most countries encourage rainwater harvesting, offering rebates and tax incentives for system installation, others impose restrictions on the maximum volume of rainwater households can collect. Imposing of restrictions is often based on water rights laws and concerns over how rainwater collection might impact the local watershed.
- In Colorado, residents are only allowed to collect up to 110 annual gallons of rainwater from their roofs for use on their property. In Utah, residents can harvest rainwater, but they must register their rainwater collection system with the state if it exceeds a specific size. In Nevada, rainwater collection is primarily limited to those who hold water rights. The state views rainwater as part of the greater watershed, meaning that restrictions are in place to protect the broader water system as set forth by Assembly Bill AB138.
Introduction
Rainwater harvesting involves collecting and storing rainwater, typically from rooftops, into tanks or reservoirs for future use. This stored water can be utilized for various purposes, including watering plants, irrigation, watering livestock, keeping pond levels stable, flushing toilets, and even drinking water if appropriately treated. Many systems collect water from rooftops, directing it into a tank or cistern next to a home or other large structures where it is stored for later use. The practice of rainwater harvesting, which is as ancient as human civilization itself, is experiencing a resurgence as a sustainable solution to water management challenges.
By collecting rainwater, we capture a portion of this precipitation before it hits the ground, thus reducing runoff and its associated problems. This intervention allows us to immediately and directly use the rainwater, easing the demand for groundwater and surface water resources.
There are different categories of rainwater harvesting systems, briefly explained below.
Roof catchments.
Roof runoff from buildings is usually of a higher quality than surface runoff and can often be used without treatment. To collect rainwater from roof catchment, gutters, down-pipes, and ground storage tanks are required. It is possible to provide a mechanism to flush out the water from the first flush rain to reduce the pollution of stored water by dust, leaves, and bird droppings washed from the rooftop into the reservoir. Surface tanks or underground tanks can be used for storage depending on the local situation.
Rock catchments.
Rock catchment is a rainwater-harvesting mechanism in areas where there is massive rock outcrop along the line of surface water runoff from precipitation. It usually provides relatively fresh and relatively less turbid water, which eventually requires relatively less treatment to render the water potable. The design of a rock rainwater harvesting system requires considering site suitability (geology, topography, rainfall), water demand, system components, water quality management, and socio-economic factors. Unlike rooftop systems, which use artificial catchments, rock systems utilize naturally impermeable rock outcrops to generate runoff.
Surface runoff harvesting i.e. stormwater collection.
Stormwater collection and reuse is becoming more broadly used due to the increase in impervious surfaces. This method involves collecting rainwater from surfaces like land, gardens, and paved areas. The collected runoff water can be stored in ponds, reservoirs, or underground tanks. This type of system helps conserve water and prevents soil erosion and flooding caused by surface runoff. Plastic sheeting (dam liners) may also be used. This type of catchment can be expensive and is clearly open to pollution. Major ground catchments are drainage basins, which naturally exist between watersheds and the runoff water is drained into dams, sand dams, check dams, ponds, etc. The amount of water collected from such catchment is dependent on the size of the area, the intensity of the precipitation, the permeability of the soil formation, the amount of absorption, and the rate of evaporation in the area.
Rainwater harvesting detention methods.
Rainwater harvesting detention refers to techniques that allow rainwater to infiltrate into the ground and augment groundwater directly. This method includes practices like creating percolation pits, trenches, and recharging wells. These structures capture rainwater and allow it to percolate into the soil, restoring the groundwater table. In-situ methods are crucial in areas with declining groundwater levels and can be integrated into both urban and rural landscapes.
Components of a Rainwater Collection System
A rainwater collection system consists of several key components, each playing a crucial role in efficiently capturing, storing, and using rainwater. Understanding these components is essential for anyone considering a rainwater harvesting system.
Catchment area.
The catchment area of a rainwater harvest system is the primary surface where rainwater is collected. In urban settings, rooftops serve as the most common catchment area. They offer a wide, unobstructed surface that is ideal for capturing rainwater. In rural or open land areas, the catchment can also include ground surfaces like fields or landscaped areas specifically designed for rainwater collection. The efficiency of a catchment area depends on its size, the material it is made from, and its cleanliness.
Conveyance systems.
Once rainwater hits the catchment surface, it must be directed to the storage vessel. This is where the conveyance system, consisting of gutters and downspouts comes into play. Gutters line the edges of the rooftop, collecting the water and leading it to downspouts. These downspouts then channel the water down and away from the building structure, often into a filtration system before it reaches the storage tanks.
Storage vessels.
Storage vessels are perhaps the most critical component of a rainwater collection system. These can range from simple barrels to large rainwater storage tanks. The choice of storage facility depends on the intended use of the collected water, the amount of rainfall, and the space available.
Water treatment and distribution.
Before the harvested rainwater can be used, it often requires treatment to ensure it is safe for its intended use. This treatment can include rainwater filtration systems to remove debris and disinfection systems to eliminate harmful microorganisms. After treatment, the water is distributed through pumps and pipes, for indoor or outdoor use.
Design and Selection of Rainwater Harvesting Systems
Designing a rainwater harvesting system involves several crucial steps to ensure the system is efficient, effective, and tailored to the project’s specific needs. Here are the key steps to consider when designing a rainwater harvesting system.
Define purpose and estimate capacity.
Understanding usage. The first step in designing your water storage tank is to clearly define its purpose. Whether for drinking water or irrigation, understanding the intended use is crucial.
Water needs assessment. Evaluate your water needs and determine how much water you require for various purposes like gardening, flushing toilets, or laundry. This assessment will help you size your rainwater harvesting system appropriately.
Storage duration considerations. Consider backup needs and seasonal rainfall trends. In arid regions, tanks may need to hold several weeks’ worth of water to ensure a reliable supply during dry spells. Design experts should utilize local climate data and historical rainfall patterns to recommend optimal tank sizes, ensuring a dependable water supply year-round.
Rainfall data. Local intensity and annual precipitation (mean, seasonal patterns) dictate potential water yield and required storage size.
Catchment area. Size, material (roof type, paved area), and runoff coefficient (losses) determine the volume and quality of water collected.
Assess site and location factors.
Soil profile analysis. Conducting a thorough geotechnical analysis is essential to evaluate soil type, groundwater levels, and load-bearing capacity. Soils with high clay content or loose structures may require additional stabilization measures to ensure longevity and integrity of your tank.
Drainage solutions. Selecting a location away from flood-prone areas is vital. Proper perimeter drainage mitigates hydrostatic pressure during heavy rainfall, protecting the tank’s integrity.
Accessibility and obstruction management. Choose a tank location that does not interfere with building foundations, tree roots, or utility lines, ensuring easy access for maintenance and inspections. Identifying the best site layout is vital for optimal functionality and future access.
Select the right material for your water tank.
Choosing the appropriate material is vital for the tank’s longevity and functionality.
- Reinforced concrete. Ideal for large-capacity tanks or those under heavy loads, such as driveways.
- HDPE (high-density polyethylene). Lightweight and corrosion-resistant, suitable for smaller residential applications.
- Stainless or galvanized steel. Durable but requires anti-corrosion treatment; best for industrial or potable water storage.
Waterproofing and leak prevention strategies.
Implement a multi-layer protection strategy by combining external coatings (like bitumen or epoxy) with internal liners (PVC or rubber). Proper sealing of seams and joints with waterproof sealants is essential to prevent leaks and ensure long-term durability.
Structural and load considerations.
Optimizing tank shape. Cylindrical tanks provide better stress distribution in high-pressure environments, while rectangular designs may be more suitable for urban settings where space is limited.
Traffic load design. If the tank is installed beneath driveways or parking lots, it must be engineered to withstand both dynamic and static vehicle loads.
Backfill engineering techniques. Utilize compacted layers and follow recommended backfill specifications to ensure stability and prevent shifting.
Maintenance access and safety features.
Incorporate manholes and inspection ports with corrosion-resistant covers. Ensure built-in ladders or step-irons are available for safe entry. Proper air vents are necessary to prevent vacuum pressure and allow gas dissipation, enhancing safety during maintenance.
Ventilation and animal intrusion control.
Install screened vents to prevent insects and rodents from entering the tank. Use lockable, gasketed access hatches to maintain water hygiene and deter tampering, ensuring the quality of stored water.
Foundation and floatation control.
A stable bedding layer, typically 100 – 150mm thick, is essential for uniform support and drainage. In areas with a high water table, consider anti-buoyancy systems such as ground anchors or concrete tie-downs to prevent flotation.
Ensure water quality and hygiene.
Utilize UNBS certified food-grade materials for potable water. Incorporate filtration systems, including pre-tank filters and post-tank treatments like UV or activated carbon, depending on the intended use to maintain water quality.
Temperature control for water tank design.
For tanks storing potable water in extreme temperatures, consider thermal insulation. Installing tanks deeper (e.g., over 1.2 meters) can help regulate temperature naturally, protecting the water from extreme heat.
Hydrostatic pressure and buoyancy management.
Design must account for both internal water pressure and external groundwater pressure. Reinforced walls and bases are necessary for tanks located in flood-prone areas to ensure structural integrity.
Piping and hydraulic design considerations.
Ensure efficient flow by strategically placing pipes to prevent sediment buildup and allow for complete tank emptying. Include anti-siphon and backflow devices to protect potable systems from contamination, maintaining water safety.
Comply with local regulations.
Adhere to local building codes, line ministry standards, and WHO guidelines for water quality and structural safety. Conduct an environmental review to assess groundwater protection and stormwater impact, ensuring compliance with all regulations.
Design for long-term durability.
Aim for an extended lifespan with minimal intervention by using durable materials and protective coatings. Implement corrosion control measures, especially in saline or acidic soil conditions, to extend the tank’s life.
Budget and return on investment (ROI).
Consider your budget. Rainwater harvesting systems can vary significantly in cost based on size and complexity. Determine what you can afford and choose components and systems accordingly. There are systems for every budget, and experts can work with you to determine the most appropriate design for your needs.
Maintenance and upkeep.
Understanding the maintenance requirements is key. Regular cleaning, system checks, and potential repairs are essential. Ensure you have the time and resources to maintain the system properly.
Installation Process for Rainwater Harvesting Systems
Once the design is finalized, the next step is installation.
- Catchment surface. This process starts with setting up the catchment area, which, in most cases, is the existing rooftop. Gutters and downspouts must be installed or modified to effectively channel the water into the storage system. Care must be taken to ensure these components are correctly aligned and secured to prevent leaks and blockages.
- Tank base. For HDPE (plastic) tanks, the tanks should be provided with full bottom support, preferably on a concrete base. The tank base must be stable enough to withstand weight of the fully loaded tank.
- Storage tank. The storage tank, which can be above or below ground, should be positioned considering accessibility for usage, maintenance, and safety.
- Inlet connection. Divert existing downpipes into the tank inlet, installing a leaf filter (rain heads/leaf eaters) and a first flush diverter to improve water quality. Where possible, include rainwater filters.
- Overflow and outlets. Install an overflow pipe near the top to direct excess water safely away, and fit outlets (tap/pump connection) at the bottom.
- For potable use, additional filtration and disinfection (reverse osmosis, UV light, chlorination) are necessary. Installing a filtration and purification system is critical to ensure the water meets required standards. Additionally, sand biofilters can be incorporated for biological treatment and charcoal filters to remove odor and dissolved gases.
Types of Rainwater Harvesting Tanks
Water tanks come in various sizes, shapes, and materials to suit different needs. The choice of a water tank depends on several factors, including the volume of water to be stored, the available space, and the intended use. Some common types of water tanks include.
- Reinforced concrete cement (RCC) tanks.
- HDPE (high-density polyethylene) tanks i.e. plastic tanks.
- Stainless steel tanks (smart tanks).
- Ferro cement tanks.
- Water jars.
- Rain barrels.
- Tote tanks (intermediate bulk containers, IBCs).
- Interlocking stabilized soil block tanks (ISSB tanks).
- Collapsible or flexible tanks used in emergency situations i.e. onion tanks and bladder tanks.
- Corrugated iron tanks.
- Bamboo tanks.
- Dammed storage (including earth dams, concrete lined ponds, PVC lined ponds, water pans).
Rainwater harvesting tanks can be either surface (aboveground) or subsurface (underground) cisterns.
Aboveground water tanks.
Reasons for choosing aboveground water tanks.
Lower upfront and installation costs. Aboveground tanks are significantly cheaper to install because they require minimal site preparation and no extensive excavation. This keeps labor costs low and often allows for quicker installation.
Easy access for maintenance and repairs. The full visibility and accessibility of aboveground tanks make routine inspections, cleaning, and repairs simple and cost-effective. Issues like leaks or damage can be quickly identified and addressed.
Versatility and variety. These tanks come in a wide array of shapes (round, slimline, vertical, horizontal), sizes, and materials (polyethylene, steel, fiberglass) to suit different site conditions and aesthetic preferences.
Portability. Lighter materials like plastic or poly mean these tanks are often easier to move to different locations if needs change
Gravity fed water pressure. If installed in an elevated position (on a stand or tower), they can utilize gravity to provide sufficient water pressure for distribution, which is useful for areas without pumps.
Flood resilience. In areas prone to flooding, aboveground tanks can be a safer option as they are less likely to float away.
Underground water tanks.
Reasons for choosing underground water tanks.
Space saving. An underground water tank does not occupy any aboveground space, making it ideal for properties with limited room. This feature allows you to utilize the land for other purposes, such as gardening or creating recreational areas.
Aesthetic appeal. One of the most significant benefits of an underground water tank is that it does not interfere with the visual appeal of your property. Whether you have a landscaped garden or a well-maintained lawn, an underground tank remains hidden and does not take away from your property’s beauty.
Safety and protection. Underground water tanks offer protection from contaminants and temperature fluctuations. Being buried beneath the ground helps keep the water safe from external pollutants, dirt, and sunlight, ensuring that the stored water remains clean and safe for use.
Durability. Because underground tanks are protected from external elements, they tend to last longer. The soil around the tank acts as a natural shield against wear and tear caused by weather conditions like sunlight, heat, and rain.
Cost effectiveness. Although underground tanks may have higher initial installation costs, they often result in long-term savings due to their durability and lower maintenance needs. The protection from external elements reduces the frequency of repairs and replacements.
Security. An underground water tank is safe from vandalism and tampering, as well as potential vehicle collisions.
Stores bigger volumes of water. With fewer limits on space for an underground tank, you can install a larger tank and store a larger volume of water.
Factors that Determine Cost of Rainwater Harvesting Systems
Costs for rainwater harvesting systems vary based on several factors. The different factors that influence the costs of rainwater harvesting systems include.
System size and capacity are the main factors affecting rainwater harvesting cost. Larger tanks cost more, with prices rising as storage capacity increases.
Material quality influences price and longevity. Plastic tanks are affordable, while stainless steel and concrete offer better durability.
Installation complexity varies by property. Underground systems need extensive excavation, increasing labour costs. Your property’s accessibility and infrastructure can affect the final price.
Filtration systems. If you plan to use rainwater for drinking, a filtration system is essential. These remove debris, bacteria, and contaminants, ensuring potable water quality, making it a necessary addition for residential rainwater harvesting systems.
Pumps. For effective water distribution, a pump is often required. Pumps allow pressurized water flow, which is essential for irrigation systems, household plumbing, and large-scale water storage.
Maintenance costs. Routine maintenance for a rainwater harvesting system needs to be undertaken. This includes filter replacements, tank inspections, and pump servicing to maintain water quality and system efficiency. Regular upkeep ensures longevity, preventing costly repairs or replacements over time.
Prices for rainwater harvesting tanks.
While prices of rainwater harvesting tanks differ based on brand, service provider, and quality, average price ranges for various rainwater harvesting products in Uganda are as provided in the table below.
Rainwater Harvesting Product | Price Range (UGX) |
Rain barrel (250L) | 70,000 – 90,000 |
Tote IBC (1,000L) | 160,000 – 180,000 |
HDPE plastic tank 5,000L (including concrete support base) | 2,500,000 – 3,500,000 |
HDPE plastic tank 10,000L (including concrete support base) | 4,500,000 – 5,500,000 |
HDPE plastic tank 24,000 (including concrete support base) | 17,000,000 – 20,000,000 |
Ferro cement tank (10,000L), aboveground | 4,000,000 – 5,000,000 |
Ferro cement tank (20,000L), aboveground | 5,000,000 – 6,000,000 |
Reinforced concrete tank (30,000L), underground | 9,000,000 – 11,000,000 |
Reinforced concrete tank (50,000L), underground | 18,000,000 – 20,000,000 |
ISSB tank 10,000L (aboveground) | 5,000,000 – 6,000,000 |
ISSB tank 20,000L (aboveground) | 7,000,000 – 8,500,000 |
ISSB tank 50,000L (underground) | 17,000,000 – 20,000,000 |
Stainless steel tank, 10,000L (with stand) | 9,000,000 – 11,000,000 |
Corrugated iron tank 10,000L | Generally not widely used |
Rainwater filter | 1,000,000 – 1,500,000 |
Dams (earth dam, concrete lined dam, PVC lined pond, water pan) | Varies based on capacity and material |
Maintenance of Rainwater Harvesting Systems
Over time, water tanks can accumulate sediments, algae, bacteria, and other contaminants that not only affect the taste and odor of the water but can also pose serious health risks. Here are some key tips for maintaining your water tank.
Regular inspections. Regular inspections are the foundation of effective rainwater tank maintenance. Schedule inspections at least twice a year, and check for the following:
- Signs of leaks, cracks, or damage to the tank and its fittings.
- Debris or leaves clogging the gutters and downpipes connected to the tank.
- Proper functioning of inlets, outlets, and any filters or screens.
Regular cleaning. It is important to clean your water tank at least once a year to remove sediment, algae, and any potential contaminants. Empty the tank and use a soft brush or pressure washer to scrub the interior surfaces. After cleaning, rinse thoroughly and allow the tank to dry before refilling it with clean water.
Regular water testing. Periodically test the water quality to ensure it meets safety standards for your intended uses. Common parameters to test for include pH levels, presence of bacteria, and other contaminants. Promptly address any issues identified through testing.
Prevention of algae and biofilm formation. Algae and biofilms can develop in the tank if sunlight and nutrients are present. Consider these preventive measures.
- Keep the tank covered with a well-fitted lid or mesh screen to block sunlight and prevent algae growth.
- Ensure proper ventilation to minimize the conditions conducive to biofilm formation.
Proper overflow and drainage. Ensure that the tank’s overflow and drainage systems are working correctly. Proper drainage prevents overflows during heavy rainfall and diverts excess water away from sensitive areas, avoiding potential water damage.
Replace insect mesh. Insect mesh will keep those pesky flies and other bugs and insects out of your rainwater harvesting tanks. However, this only works if the mesh is well maintained. Keeping the mesh maintained and in good condition at all times ensures the safety of your water and removes any possibility of small flies breeding in your water which eventually could end up in your house/building’s water supply.
Check pipes and pumps. An effective rainwater tank maintenance procedure you should take is to check the pipes and pumps that run from your collection tank into your house/building. Any cracks or broken pipes could be leading to not only a loss of water, but an entry point for viruses and other diseases to thrive. Replace any pipes and pumps that are no longer at their best quality to prevent harmful bacteria being prevalent in your vicinity.
Trim overhanging branches. Trees near your roof can drop leaves, bark, and even branches onto your catchment surface. Keep branches trimmed back to reduce debris and prevent damage during storms.
Common Mistakes to Avoid in Rainwater Harvesting
Not cleaning the roof. When collecting rainwater, you need to make sure the water is clean and free of contaminants. The roof is the surface from which you collect rainwater, so it is important to clean your roof regularly to remove dirt, debris, fallen leaves, and bird droppings. If you don’t clean your roof regularly, these contaminants can end up in the water you collect, which can make the water unfit for certain uses.
Using inappropriate materials for water storage. The choice of storage container is important for the quality of the water you collect. Water storage tanks must be made of plastic or concrete. Metal containers can rust and contaminate water with heavy metals. It is also important to choose a storage tank size adapted to your consumption to avoid the risk of water stagnation.
Not filtering rainwater. Even if you clean your roof regularly, there are still contaminants in rainwater, such as air pollution, bird droppings, or airborne particles. It is therefore important to filter rainwater before storing it. The absence of a filter allows these contaminants to end up in the stored water, which can make it unsuitable for certain uses.
Not complying with regulations. When collecting rainwater, it is important to comply with local regulations regarding water collection, storage, and use. These regulations may vary by region and country, but in general, they address issues such as the location of the storage tank, the size of the tank, the quality of the water stored, and the use of the water collected. Failure to comply with these regulations may result in fines or other penalties.
Using rainwater for inappropriate uses. Although rainwater harvesting can be useful for many uses, it is not suitable for all situations. For example, rainwater should never be used for drinking or cooking unless it has been treated to remove all contaminants. Similarly, rainwater may not be suitable for certain types of plants or industrial uses. It is therefore important to check the suitability of rainwater for each use.
Missing or too small overflow. Rainwater tanks should have an overflow pipe added to the container. An overflow pipe gives an outlet for incoming rainwater when the container is full. Overflows prevent excess rainwater from flowing out of the manway or backing up inside the inlet. The overflow pipe should be at least the size of the tank inlet pipe and should be positioned below the tank inlet pipe. It should be plumbed to discard excess water away from the bottom of the tank and away from nearby structures.
Tapping the water storage tank less than 6 inches above the base of the tank. Install an outlet, hose bib, or distribution plumbing at least 6 inches from the tank base to avoid drawing water from the sediment layer, anaerobic zone, and biofilm region. It is this area where unfiltered materials and microbiological activity can be found and are most prevalent.
Improper dissipation of pressure by inlet pipe. Inlets to storage tank must be designed to dissipate pressure of influent stream and minimize the stirring of any settled solids. Consider using a calming device for rainwater as it flows into the tank. This can be either an upward-curved inlet pipe or an accessory such as a solar shield for the manway. Calming the incoming water during rain events will help to prevent disturbing the bottom anaerobic zone and remixing of any sediment that previously settled.
Using clear, entirely translucent plastic containers for regular, repeat collection and use of rainwater. Clear water containers can be prone to high levels of algae with sunlight exposure. Algal growth is an unwanted condition that can affect water quality, clog piping, hoses, or equipment and will increase the amount of maintenance. Black and dark green water tanks are opaque and can completely block incoming sunlight, hence recommended.
Use of containers previously used for chemicals. Do not use any container for rainwater harvesting that was previously used to store chemicals. Many chemicals and materials can get absorbed into a container and release out again into water over time, even if the container is washed and cleaned many times. Brand new rain tanks or IBC totes that are new are recommended for rainwater harvesting.
Drinking rainwater without purification. Do not drink rainwater collected from a roof unless it has been properly treated (disinfected/sanitized) against potentially harmful microorganisms. While rainwater is fairly clean, rain can collect dirt, dust, and filth such as animal fecal matter as it flows over a roof and through gutters. This can be a source of bacteria, parasites, and sometimes viruses that can cause illness.
Conclusion.
The process of rainwater harvesting is not as simple as simply collecting rainwater in a storage tank. It demands careful planning, precise execution, and regular maintenance to avoid common pitfalls and ensure the system functions efficiently. SANKOFA, backed by a proven solid track of successful rainwater harvesting projects, offers expert guidance, ranging from consultation, design and product selection, and installation. As a leading company in design and installation of rainwater harvesting tanks, we do ensure that each project we execute adheres to the utmost standards of excellence and industry best practices.