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On average, an aquaponic system uses 1-3 percent of its total water volume per day, depending on the type of plants being grown and the location. Water is used by the plants through natural evapotranspiration as well as being retained within the plant tissues. Additional water is lost from direct evaporation and splashing. As such, the unit will need to be replenished periodically. The water source used will have an impact on the water chemistry of the unit. Below is a description of some common water sources and the common chemical composition of that water. New water sources should always be tested for pH, hardness, salinity, chlorine and for any pollutants in order to ensure the water is safe to use.
Here it is important to consider an additional water quality parameter: salinity. Salinity indicates the concentration of salts in water, which include table salt (sodium chloride - NaCl), as well as plant nutrients, which are in fact salts. Salinity levels will have a large bearing when deciding which water to use because high salinity can negatively affect vegetable production, especially if it is of sodium chloride origin, as sodium is toxic for plants. Water salinity can be measured with an electrical conductivity (EC) meter, a total dissolved solids (TDS) meter, a refractometer, or a hydrometer or operators can refer to local government reports on water quality. Salinity is measured either as conductivity, or how much electricity will pass through the water, as units of microSiemens per centimetre (µS/cm), or in TDS as parts per thousand (ppt) or parts per million (ppm or mg/litre). For reference, seawater has a conductivity of 50 000 µS/ cm and TDS of 35 ppt (35 000 ppm). Although the impact of salinity on plant growth varies greatly between plants (Section 9.4.2, Appendix 1), it is recommended that low salinity water sources be used. Salinity, generally, is too high if sourcing water has a conductivity more than 1 500 µS/cm or a TDS concentration of more than 800 ppm. Although EC and TDS meters are commonly used for hydroponics to measure the total amount of nutrient salts in the water, these meters do not provide a precise reading of the nitrate levels, which can be better monitored with nitrogen test kits.
Collected rainwater is an excellent source of water for aquaponics. The water will usually have a neutral pH and very low concentrations of both types of hardness (KH and GH) and almost zero salinity, which is optimal to replenish the system and avoid long-term salinity buildups. However, in some areas affected by acid rain as recorded in a number of localities in eastern Europe, eastern United States of America and areas of southeast Asia, rainwater will have an acidic pH. Generally, it is good practice to buffer rainwater and increase the KH as indicated in Section 3.5.2. In addition, rainwater harvesting will reduce the overhead costs of running the unit, and it is more sustainable.
The quality of water taken from wells or cisterns will largely depend on the material of the cistern and bedrock of the aquifer. If the bedrock is limestone, then the water will probably have quite high concentrations of hardness, which may have an impact on the pH of the water. Water hardness is not a major problem in aquaponics, because the alkalinity is naturally consumed by the nitric acid produced by the nitrifying bacteria. However, if the hardness levels are very high and the nitrification is minimal because of small fish biomass, then the water may remain slightly basic (pH 7-8) and resist the natural tendency of aquaponic systems to become acidic through the nitrification cycle and fish respiration. In this case, it may be necessary to use very small amounts of acid to reduce the alkalinity before adding the water to the system in order to prevent pH swings within the system. Aquifers on coral islands often have saltwater intrusion into the freshwater lens, and can have salinity levels too high for aquaponics, so monitoring is necessary and rainwater collection or reverse osmosis filtration may be better options.
Water from the municipal supplies is often treated with different chemicals to remove pathogens. The most common chemicals used for water treatment are chlorine and chloramines. These chemicals are toxic to fish, plants and bacteria; these chemicals are used to kill bacteria in water and as such are detrimental to the health of the overall aquaponic ecosystem. Chlorine test kits are available; and if high levels of chlorine are detected, the water needs to be treated before being used. The simplest method is to store the water before use, thereby allowing all the chlorine to dissipate into the atmosphere. This can take upwards of 48 hours, but can occur faster if the water is heavily aerated with air stones. Chloramines are more stable and do not off-gas as readily. If the municipality uses chloramines, it may be necessary to use chemical treatment techniques such as charcoal filtration or other dechlorinating chemicals. Even so, off-gassing is usually enough in small-scale units using municipal water. A good guideline is to never replace more than 10 percent of the water without testing and removing the chlorine first. Moreover, the quality of the water will depend on the bedrock were the initial water is sourced. Always check new sources of water for hardness levels and pH, and use acid if appropriate and necessary to maintain the pH within the optimum levels indicated above.
Depending on the type of filtration (i.e. reverse osmosis or carbon filtering), filtered water will have most of the metals and ions removed, making the water very safe to use and relatively easy to manipulate. However, like rainwater, deionized water from reverse osmosis will have low hardness levels and should be buffered.
Source: Food and Agriculture Organization of the United Nations, 2014, Christopher Somerville, Moti Cohen, Edoardo Pantanella, Austin Stankus and Alessandro Lovatelli, Small-scale aquaponic food production, http://www.fao.org/3/a-i4021e.pdf. Reproduced with permission.