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Environmental Considerations for Planning Rain Water Harvesting Systems

 

The need for rain water harvesting has gained increasing importance, as problems of groundwater depletion, and its deleterious consequence, have surface in different part of the world. Several methods for recharging rain water as ponds, recharge pits/infiltration well, etc are being popularly practiced at local/peri-urban level in India.

 

From a utility point of view optimal location & optimum capacity based on the anticipated use of harvested water are the key consideration in siting, designing and constructing any RWH structure. At a community level, the siting of the structure has to ensure that natural drainage is not disrupted and the costs of conveying storm water are kept at a minimum. This requires utilization of spaces/areas where water’s natural course flows or accumulates.

 

A very low infiltration rate can be detrimental to water harvesting systems because of the possibility of water-logging. On the other hand, a low infiltration rate lead to high runoff, which is desirable if the water is to be conveyed and harvested at a different location. Compacting of medium in a rain water harvesting system as a result of heavy traffic both from machinery or natural compacting under load could also result in lower void ratio, and hence, lower storage capacity for harvested water. Hence, the structures need to be sited, designed and constructed in a way so as to avoid any such influence. Further an optimum technology for RWH would be the one which can harvest maximum possible rain water for a particular capacity and/or cost.

 

From the perspective of rain water harvesting system, it is desirable that the storage capacity of the system is sufficient to release water to the ground at an adequately slow rate to take care of reduction infiltration capacity of the soil due to compaction, capping/sealing or saturation.

Although the communities at large are aware about the above listed considerations owing to traditional practice of these systems, there is limited awareness on environmental considerations required to be taken into consideration for adequate design and construction of these structures.

 

The environmental considerations vary based on the anticipated use of harvested water. There are two primary uses of harvested rain water reuse and recharge of ground water. Although the option of reuse can provide immediate and conspicuous benefit to the building residents/society, there is usually a constraint to the extent of storage that can be provided for storing the harvested water. The other concern is the perceived quality of the harvested rain water. A number of studies, revived by Gould(1999) and Lye (2002), have identified various pathogens including Salmonella, Shigella, Vibrio, Clostridium, Crypylobacter, Cryptosporidium and Giardia specially in tank water sample. In contrast with these examples, others have reported roof-harvested and tank-stored rain water to be of acceptable quality for drinking and cooking purposes (Dillaha and Zolan 1985), presenting no increased risk of gastro-intestinal illness on consumption when compared with chlorinated and filtered public mains water (Heyworth 2001).

 

Thus, a clear consensus on the quality and the health risk associated with roof collected rainwater has not been reached. This has increased the perceived risk with the use of rain water for potable and other non-flushing uses and the use of harvesting water for ground recharge is becoming more and more popular. However, blindly implementing rain water harvesting without proper site assessment can have serious repercussions. Rain water is slightly acidic and very low in dissolved minerals; as such, it is relatively aggressive.

 

Rain water can be dissolve heavy metals and other impurities from materials of the catchment and storage tank. During its replenishment, the ground water is vulnerable to contaminants that may be in the surface water or soil. The aquifer may be protected from surface water contamination by the presence of underground deposits of impervious materials such as clay or bedrock. For example, a sanitary landfill constructed in sand and gravel deposits is more likely to lead to groundwater contamination than a landfill constructed in clay deposits. Even when RWH structures are not plan in contaminated areas, natural percolation of rain water still poses of threat of groundwater contamination.

 
Method Cost per Volume of Water Harvested Void Ratio (Effective Storage for Recharge/Utiliazation Later) Ease of Installation Dedicated Land Allocation Siting Constraints Maintenance Requirements SkillnRequired for Maintenance Loss of Harvested water Aesthetic Appeal Quality of Harvested water
Ponds Low The entire volume of pond is available for storage N.A. Required More Prone to accumlation of debris and slit. May Require de-silting or dreadging on yearly basis High High on sunny days Good (if mainained) Doubtful. Harvested water can either be recharged to ground or used for horticultute. For domestic application treatment is require.
Recharge pit Low 40-50% Skill required to install a recharge pit. Required less Needs to be cleaned annually or biannually high nil low Water is recharged into ground and is not accessible for reuse or for direct applications to plants.
Water storage tank High The entire volume of the storage tank is available Skill require to construct concrete tank The space on tank can be utilized for purposes suc as parking/storage racks but the slab has to be designed to take care of the load having implications on already high cost. More low low No loss of harvested water after storage nil The water once stored is not prone to contamination.
Plastic crate system low 95% void ratio Low The space on top can be utilized for any purpose as a virgin land Very less Low low No loss of harvested water after storage Nil The water once stored is not prone to contamination and is available for both recharge and reuse.

Groundwater contamination presents very complex issues. Control of ground water contamination is depend on 4 interrelated systems: regulation, design, monitoring and remediation. Regulations intended to control ground water contamination are very difficult to enforce. Remediation or cleanup of a contaminated aquifer through air stripping, a process by which water is removed from an aquifer, treated to removed contaminants, then return to the aquifer can only be perform in limited situations due to obvious cost/technology constraints. Design and monitoring requires tapping of entire contaminated catchment which is significantly cost intensive.

 

One innovative concept which attractively addresses the above mention criteria and constraints is plastic crate RWH system. In this system, crates are stacked one on top of another and enclosed by a water penetration or a water tight sheet depending on whether the tank formed by stacking of cracks is to be used as a storage tank or as a porous storage to release water gradually to enhance recharge. The system allows for integration with a treatment scheme to take care of concern related to quality.

 

Figure 1 depicts a sample arrangement which can potentially be used for not only remediation of contaminated soils but also to track the contaminated subsurface flows from reaching the precious ground water aquifers. The structure can track the contaminated subsurface flow and the collected contaminated sub surface flow can be treated in and effluent treatment plant for adequate treatment before further reuse/discharge. The effluent can be tapped to through the orifice (which can be planned at the bottom to rapidly convey the contaminated runoff to the ETP). The inlet can be used to inject treated water for dilution or neutralizing agents if required. The overflow takes care of excess flow by diverting the same to a reservoir or directly to ETP.

 

Apart from envisaged uses, such systems have other potential advantages. These system can be use to save money on large projects by reducing the cost of one of the key elements, the storage tank, using and interlinked plastic lattice below below ground level that can be sized to fit and be placed just about anywhere. In the long run, this system is a lot cheaper than building a concrete tank onsite or bringing in a poly tank or fiberglass tank. The crate systems can be put under driveways or high-traffic areas, versus some structures that cant be put in those locations