Reuse of Wastewater in Agricultural & Industrial Application

Agricultural Practices

crop farming using wastewater
Reuse of wastewater for crop irrigation must not pose health risks both to the plantation workers and the consumers and special attention needs to be given to check on whether high coliform bacteria concentration still remains in the effluent. Despite being exposed with potential health risk, the practise on reuse can never be entire eliminated especially with the constraints in getting continuous fresh water supply and partly also due to the abundance of nutrient present in the treated wastewater. Farmers around the world have been relying on this reclaimed wastewater to grow crops and reduce the risk of failures resulting from drought and to cut down cost of applying fertilizers. Overall, the suitability, apart from factors related to crops tolerance should mainly focus on safety issues especially on whether pathogens and presence of hazardous substances still remains after the treatment plant removal processes. Because of that, governing bodies including WHO have set certain rules and guidelines including follow-up recommendation and restriction on possible reuse.

Toxicity is mainly due to presence of heavy metals. Irrigation with wastewater contaminated with heavy metals not only reduce crop yield but also the accumulation will eventually passed on to consumers. Issues related to specific ion toxicity can also remain a problem especially when consumers and households’ cleaning chemicals contain elevated levels in certain ion content especially boron and this substances eventually remains in the wastewater. The toxicity effect can actually end up harming the human population in two ways, either by contamination brought about when leachate pollutes the groundwater source or when the toxic heavy metals remain in the crops. Thus a proper monitoring plan needs to be set up to ensure that the wastewater does not exceed the maximum recommended limit and in-detailed studies need to be conducted to evaluate the overall effects.

Microbial Contamination
Health hazards can also be caused by microbial contamination but risks can be mitigated depending on crop type, irrigation method and also by applying safe farming work practices. In fact, WHO has set a risk assessment and management guidelines to tackle on this issue while governing bodies also spelled out their own reclamation regulations before the wastewater can be applied in irrigation practices. For instance, the most common approach is based on median number of total coliform count and in order to achieve this level, a tertiary treatment processes using filtration or ozone disinfection needs to be applied. Usually the minimum treatment requirement is based on the use of reclaimed wastewater and its applications. Apart from microbial, sometimes infectious agents such as worms and trichuris can also pose serious health risk, thus all these also needs to be taken into careful planning.

Industrial Application

Demands of water for industrial application have risen significantly over the last decade and in fact, most of it is largely used as cooling tower make-up water which is needed in almost every industry. Cooling tower basically operate in a closed system separate from the main processes, thus use of treated wastewater presents an opportunity to reduce the consumption of potable freshwater. Usually before the effluent is used, it will be subjected to chemical treatment to adjust the pH, addition of scaling and corrosion inhibitor chemicals so that this will reduce scaling and corrosion effect to the systems. Although use of freshwater can still cause all of these problems that reduce the efficiency of the whole cooling tower operation, the effect is more profound when reclaimed wastewater is used here.

Scaling and Corrosion
Scaling is basically caused by formation of deposits mainly calcium and magnesium scales. In order to reduce this effect, phosphate should be removed in the upstream wastewater treatment processes so that formation of the first calcium salt which is calcium phosphate can be controlled. There are also other wastewater treatment options such as ion exchange that basically reduce the calcium and magnesium content but however, the high treatment cost factor will not make the whole approach on using treated wastewater seem not feasible. Similarly, for wastewater with still high TDS level will increase the electrical conductivity and therefore lead to accelerated corrosion effect. Presence of metals like iron will further make it worse and usually treated wastewater must undergo addition of inhibitors before it can be transferred for use in the cooling water system.

Biological Hazard
Use of reclaimed wastewater that contain high coliform count can also lead not only towards reduced heat transfer efficiency but also other health related issues as loss of water during normal operation can create airborne effect that can jeopardize the health of the workers operating the cooling tower. Even though most wastewater from treatment plant has already undergone disinfection procedure, but however, due to the ideal conditions present in the system, whereby it is moist, rich in nutrient and organic content, this will promote biological growth. To deal with the problem here, biocide is usually added in proportionate amount and with wastewater source; do expect to use higher dosage.

Recommended Engineering Books