It is preferable to filter the rainwater before storing it. If leaves and other organic material enter the storage tank, they decompose and support bacterial growth in the tank. Dirt and other debris, if not filtered out, can cause blocks in the plumbing system when the stored rainwater is used. Different filters exist: some are commercially available while others can be put together by you.
Care should be taken to maintain the cleanliness of the roof. It is advisable to cut tree branches that overhang on the roof. This would not only reduce the leaves, flowers, fruits and bird droppings that fall on the roof but would also reduce access to squirrels and rodents.
Filters introduce some losses into the rainwater collection system and the filter efficiency can either be calculated for simple systems or will be stated by the manufacturer.
Primary filters / leaf guards
The first level of filtration could be a grating at the outlet of the catchment or the inlet of the gutters or downtake pipes to prevent large coarse debris like leaves from entering the rainwater transportation and storage network. For open gutters, leaf guards which are usually ?” mesh screens in wire frames may be installed along the length of the gutter. The rooftop must be regularly cleaned for the leaf screens to be effective, else the piled up leaves will clog the screen and prevent rainwater from entering the gutters or downtake pipes. This can even result in leakage of water from the roof. If wire meshes or gratings are not used at the outlet of the catchment, the filtration system installed should be able to segregate such debris from the collected rainwater.
Secondary filters
The second level of filtration is required to remove finer particles and even bacteria, so that the collected rainwater is free from suspended solids and organic contaminants. Some filtering technologies are described and compared below. The final choice of a filtration system depends on the cost of the device and maintenance needs. Off-the-shelf availability is also a deciding factor since not many people will make their own filtration systems.
Variations to the sand –gravel filter include using a sponge layer on top to filter out coarse debris or adding a layer of charcoal or activated carbon to improve the odour.
Maintenance: The top layer of sand to a depth of about 3 cms (or sponge if used) needs to be cleaned periodically. The sand or sponge can be removed and soaked /cleaned in a bucket of water and replaced. There could be fungal growth on the sponge if prescribed maintenance is not followed. If charcoal is used, it needs to be changed every year. It is also advised to clean the meshes and the top layer of gravel.
VARUN: is a slow-sand filter constructed in a 90 litre (HDPE) drum. The lid is turned over and holes are punched in it. This is the first sieve which keeps out large leaves, twigs etc. Rainwater coming out of the lid sieve then passes through three layers of sponge and a 150 mm thick layer of coarse sand. The filter removes suspended solids from the harvested rainwater. It has been developed by S Vishwanath, a Bangalore based water harvesting expert. According to him, from a decently clean roof 'VARUN' can handle a 50 mm per hour intensity rainfall from a 50 square metre roof area. Based on these calculations, when a rainwater harvestingsystem is being designed for a new house, the optimal number of filters can be recommended.
Maintenance: The sponge needs to be cleaned periodically and the top layer of sand to a depth of about 3 cms needs to be cleaned at the end of a rainy season.
figure 1: Slow Sand FilterCost: A filter that can service a rooftop area of 100 m2 costs Rs 4500/-. (Price in Bangalore in 2006)
figure 2: Slow Sand Filter Crossection
Implementation precautions of sand filters
Variations of the sand and gravel filters have been tried by many rainwater harvesters. Some precautions have to be taken when implementing sand filters. These are detailed below:
The advantages and disadvantages of sand filters are given in the table below.
Advantages
Disadvantages
The cost of mesh filters depends on the design apart from many other factors.
Some of the advantages and disadvantages of mesh filters are given in the table below.
Cost: (Prices in Bangalore in 2006)
Model FL-150 costs Rs 4750/- and Model FL-250 costs Rs 8750/-.
Table Comparison of sand filters and mesh filtersA comparison of sand filters and mesh filters is given in the Table below.
Sand Filters | Mesh Filters |
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First flush diverters
The first rain that falls after a long dry spell usually carries with it a lot of dirt, dust and debris that would have collected over the roof. It also dissolves many air pollutants on its way down. It is therefore a safe practice to divert the first runoff away from the storage tank. A first-flush diverter is used for this purpose. After the ‘first-flush’, water that is colleted is much cleaner. The volume of water that needs to be flushed out is a function of the number of dry days, the season, the catchment surface cleanliness and the surrounding environment. Though there are no specified standards and different studies have stated inconsistent results, a thumb rule that can be used is that for every sq mt of catchment area, 0.5 litres of water are flushed out. This is equivalent to the first ten minutes of average intensity rainfall.
Ease of operation and maintenance are key factors to effective first flush devices. Without requisite attention, these diverters may get blocked and may even pollute the water in the tank.
Though there are a number of first –flush diverter designs, one that is relatively easy to build and maintain is the standpipe that is detailed below.
The standpipe consists to fill a vertical PVC pipe perpendicular to the pipe that leads into the storage tank as shown in the figure. It has a threaded plug at the bottom. Water from the downtake pipe fills up the standpipe and when full, water is allowed to go into the storage tank. The threaded portion at the bottom must be removed after each rainfall event to drain out the water in the standpipe and keep it empty for the next rainfall. Alternately a small drip hole in the standpipe cap allows the water to leak out of the standpipe slowly. A floating ball valve arrangement can also be made. As the first flush water enters the standpipe, the ball valve rises and blocks the standpipe inlet, allowing the water to flow into the storage tank.
Source: Rainwater Harvesting - Trainers’ Manual published by Karnataka Urban Infrastructure Development and Finance Corporation