Addressing the Challenges of Removing Chemical Residue in the Water Treatment Process

Introduction

In 2019, the World Health Organization reported that a third of the global population lacks access to safe drinking water.¹ Safe drinking water is a finite resource and has been put under increasing pressure as the global population continues to grow. For instance, the global population in 2020 reached 7.8 billion and represented an 0.8 billion increase from 2010, which reflected a one billion increase from 1998 when the global population stood at 6 billion. ¹ Since the early 1980s, the global population has doubled while the available water resource has remained the same. Population explosion has increased the water demand while also releasing pollutants to the environment and also water sources. ¹ Also, industrialization has not only increased water consumption but has also led to the release of toxic chemicals, which can contaminate water resources. ¹ Additionally, agriculture has been growing in significance and some of the chemicals and fertilizers used can escape to the environment and contaminate water resources. Therefore, the increase in demand for water has occurred alongside an increase in contaminants in water sources, which traditional water treatment processes are unable to eliminate. Traditional water treatment process was developed to respond to the threat of microorganisms and may leave some chemical contaminants in water.⁴ There is a need to look into ways of addressing these chemical contaminants. This report addresses the challenge of removing chemical residue in water treatment. It explains how the traditional water treatment process is inadequate for addressing chemical residue and proposes ways of removing these pollutants.

Conventional Water Treatment Procedure

The key challenges of removing persistent chemical residue from water treatment arise due to the general orientation of the conventional water treatment procedures.³ Conventional wastewater treatment is not designed to remove chemicals from water. Instead, it was developed to remove sediments, organic matter, and microorganisms. ³ Every stage as illustrated in table 1 below is targeted towards removing solids and microorganisms, while none is dedicated for deactivation or removing dissolved chemicals residues. It aims at progressively eliminating nutrients from the water and hence making it inhospitable for this microorganism, followed by disabling the majority of remaining pathogens, through chlorination among others.

Table 1: Stages of the conventional water treatment process and its orientation towards the removal of solids and microbes. (Adapted from the Commonwealth of Pennsylvania) ²

The conventional water treatment process is dominated by mechanical and biological processes including bar screening for removal of large sediments, grit chambers for removing gritty matter, and flocculation and sedimentation for removal of nearly all suspended matter. These three steps leave the water with no nutrients, such that microorganisms cannot survive and reproduce. ³ Equally, the bulk of microorganisms is removed in the process. The final step of conventional water treatment includes chlorination or UV treatment among overs aimed at eliminating the remaining microorganisms. However, in an environment where chemical pollutants lurk in the environment and can potentially gain access to water resources, numerous chemical pollutants have been found in water sources as well as in clean drinking water. Water sources are increasingly polluted by chemical contaminants in the environment, while conventional water treatment processes are not designed to degrade or remove these chemicals.

While other methods exist that can be used to remove all pollutants from water, they remain expensive, energy-intensive, and hence inaccessible. Such include distillation and reverse osmosis. Distillation entails boiling water such that it evaporates leaving all impurities and collecting the vapor to condense it back into the water. The process eliminates nearly all the pollutants from the water. However, it is prohibitively expensive, and most cities and villages cannot find enough resources to obtain resources for setting up distillation columns that can treat enough water for all residents. Also, the process uses an unaffordable amount of energy and is impossible to sustain. Reverse osmosis entails pumping water through a semi-permeable membrane such that only water’s small molecules pass through as all impurities are ‘sieved off’. This process is also very expensive in installation and requires large sums of money to maintain. In addition, both processes use huge amounts of energy and are not environmentally sustainable. Therefore, it is justifiable to rely on the conventional approach and include additional steps that can deal with emerging pollutants than adopting new methods of water treatment altogether.

Classes and Sources of Chemicals Contaminants

Different classes of chemicals can contaminate water resources including swamps, rivers, and lakes. A notable percentage of these chemicals persist even after water treatment. Pollutants can be chemicals used in agriculture, fertilizers, or detergents.

Agrochemical residue is a ubiquitous pollutant in most water sources.⁴ Sharma and Bhattacharya noted that tackling agrochemical residues is complicated due to the wide variety of these chemicals in the application.⁹ These can include insecticides, fungicides, and nematicides among others. Some of them are water-soluble and are carried by water to different areas in the environment including swamps, rivers, and lakes, which form a key source of water for villages and cities. Equally, others persist in the environment including in water sources, and water may not be safe for consumption even if active use of these chemicals is halted. ⁹ All these chemicals have different chemistry, mode of action, and can be neutralized in specific ways. Therefore, water treatment may require correct identification of the Agrochemical residue present to inform the choice of the best procedure to degrade or remove it. As explained earlier, the conventional water treatment approach is inadequate in degrading and removing them.

Andrass Székács, et al. conducted a long-term study to trace the presence and changes in the concentration of agricultural chemicals on land and also in drinking water sources in Hungary.⁵ They conducted a survey between 1990 and 2015, which revealed that pesticides persisted in the ground as well as groundwater sources. ⁵ They also found out that these pollutants could easily get into drinking water sources. The research entailed the collection of 2000 samples on the grounds, in watercourses, and drinking water. The study established that ground and watercourses had been contaminated by agrochemicals especially herbicides used for corn production. ⁵ Importantly, the concentration varied with the active usage of these chemicals. For instance, the concentration of chemicals such as atrazine, diazinon, and trifluralin reduced and ultimately disappeared after they were prohibited across the EU. ⁵ Also, the study established that water-soluble pesticides were found to spread further from their source, and hence appeared in more samples. An increase in chemicals’ use corresponded with a commensurate increase in their concentration in watercourses and drinking water. ⁵ However, less soluble chemicals were localized and mainly persisted around their use areas, except when they were moved alongside eroded materials.

Other chemical contaminants of concern that originate from agriculture are fertilizers. These are applied to crops hundred and even thousands of times as compared to agrochemicals. To be effective, their active ingredients are highly soluble and hence highly mobile. Also, some are easily leached. While research has greatly increased efficiency in fertilizers’ application, an estimate from Colorado States University revealed that about 25% of the nitrogen applied for corn production is lost, either through leaching where it is taken to deeper layers or is converted into inert atmospheric nitrogen and released to the atmosphere.⁷ The process of leaching is of primary concern, as it delivers nitrates to water sources. Depending on the concentration, the residual amount can persist after water treatment and cause serious health problems. High concentrations of nitrates in drinking water have been linked with The Blue Baby Syndrome. ⁷ This occurs when dissolved nitrogen interferes with healthy hemoglobin function and hence curtails their capacity to carry oxygen around the body making children turn bluish. The condition may have fatal consequences if untreated. In recognition of the persistence of Nitrates after water treatment, the US Environmental Protection Agency (EPA) has set the maximum allowable limits of nitrates in drinking water at 10 milligrams per liter. ⁷ It can be expected that with the increase in agriculture and hence the use of fertilizers, the threat of nitrate contamination in drinking water is likely to expand. Phosphate too can escape into water sources from fertilizers and has the effects of making the water turbid. However, conventional water treatment procedures may adequately remove phosphates from the water.

Detergents form another crucial category of persistent chemical contaminants. They are ubiquitous as they are easily generated from day-to-day activities in nearly all households. At the same time, most people are unaware of their adverse effects on the environment. Despite the perception that detergents are generally safe, such that scanty regulations addressing their release into the environment exist, detergents contain large percentages of phosphate salts, which degrades water quality. Detergents also contain a benzene and wide array of chemicals that can be carcinogenic but are non-biodegradable. ⁹ These compounds cannot be broken down in the conventional water treatment procedures and also hamper the breakdown of organic matter, especially the stages that are dependent on the use of microbes. In addition, their non-biodegradable nature makes them persist in the environment including in water sources, which means that the threat they pose will continue to persist many years after cessation of their use in detergents.

According to Năstasă et al, NăstasăAgain, there is inadequate understanding of the dangers posed by detergents in the environment and also scanty regulations to control their release.⁸ Samples taken from major water bodies such as the Danube, Prut, and Bistrita rivers including lakes along these bodies were found to contain large percentages of ionic compounds originating from detergents, which were both noxious and carcinogenic. ⁸ Of note, these water bodies form a crucial source of drinking water and pose risks to residents. Equally, the case of these rivers is an example of other water bodies and drainage basins, some of which pass through even more densely populated settlements, and hence receive large amounts of industrial effluents containing detergents of different types. Therefore, a large percentage of the global population is exposed to residual, potentially toxic compounds from detergents.

Degrading and Removing Residual Chemicals from treated Water

To effectively remove chemical pollutants during and after water treatments, the conventional procedure needs to be expanded to include steps that can degrade or remove these pollutants. This addition may entail the inclusion of a new stage after chlorination or expanding the existing stages, especially the secondary sedimentation, which is also referred to as activated sludge. The activated sludge stage entails aerating the water during treatment to promote microbial growth to degrade organic matter. Ideally, this procedure entails balancing between time and cost; it is shortened for ordinary water. However, prolonging this stage can provide additional time for the degradation of organic compounds that happen to be microbial nutrients such as phosphate and nitrates. The concentration of nitrate can be lowered to safe levels by significantly prolonging the activated sludge stage.⁷ This can pave way for reducing the adverse effects it causes when consumed in larger concentrations. Phosphates can also be degraded by this procedure as they are nutrients that can be consumed and hence be degraded by microbes. However, the extension of the process is ineffective in eliminating chemicals that are inherently toxic to microbes including herbicides, insecticides, and fungicides among others. Therefore, these need to be addressed by using other methods.

Alongside precipitation of organic matter through the use of microbes, chemical precipitation can also be added especially for the removal of excess phosphates. ⁹ Treatment procedures for water with high concentrations of phosphates can be varied to include chemical precipitation after the secondary sedimentation state. Chemical precipitation is achieved through mixing water with calcium, aluminum, or iron ions, and sediments phosphate-based compounds which settle at the bottom and are hence removed. ¹⁰

Another method proposed for eliminating chemical residues in water is the use of specially engineered chemicals for neutralizing pollutants or accelerating their further degradation. Andrass Székács proposed testing the water to identify the specific chemical pollutant and hence determining the best compounds to be used to degrade the persistent chemical pollutant and achieve harmless byproducts. ⁵ However, the quickest and safest method of dealing with varied agrochemicals as well as detergents is the Advanced Oxidation Process (AOP). ⁹ AOP is based on the realization that all pollutants can react with oxygen to form less harmful products. Therefore pollutants are exposed to highly oxidative materials such as the ozone of Fenton.⁶ This method is effective for a wide range of chemicals but is prohibitively expensive. In addition, it may alter the final PH of the water and necessitate further remediation to make the water useable. ⁹ Importantly, it is unjustifiable when dealing with large volumes of water that have only a small concentration of contaminants. The method, is more suitable for treating water with a high concentration of contaminants, say industrial effluents, than it is suitable for being an additional stage of the conventional water treatment process. For the latter, a cheaper physical process such as adsorption becomes appealing.

The adsorption method becomes appealing for varied chemical molecules, especially as an addition to the conventional water treatment process. After the conventional water treatment procedure, over 99% of solids are already removed and the water is clear, enabling it to easily pass through porous materials for the remaining chemicals to be adsorbed. Also, the water has minimal particles that compete for adsorption sites on the treatment surface, which exposes chemical pollutants to getting closer to porous surfaces. The molecular nature of these compounds makes them be attracted and hence be attached to adsorbent material, especially activated charcoal. Therefore, water treatment plants can be expanded to integrate an additional step for the treated water to be passed through filters made of materials with molecular porosity such as activated carbon.⁹ Therefore, persistent chemical molecules can be attached to these pores which enhances the water purity. Carbon-based filters are also effective in removing nitrate from water. Nitrate ions are attached to the walls of carbon filters, which greatly reduces their concentration in water, making the water safe for consumption. However, while activated carbon is effective against selected chemical agrochemical residues, it is ineffective against phosphates from fertilizers and phosphate ions from detergents. In other instances, activated alumina can be used and produces similar effects as activated carbon. The process of adsorption forms the most cost-effective way of removing persistent chemical pollutants from treated water. It is cheap both in the initial and running cost and does not require additional energy. It is safe for and accessible for small municipalities and villages with limited resources for cleaning their water resources through other means.

Conclusion

The conventional water treatment method is inadequate in removing chemical pollutants as it is designed to address solids and microorganisms. However, population growth, industrialization, and expansion in agriculture have increased the chemical pollutants in the environment. These pollutants include agrochemicals, fertilizers, and detergents. To address these residual chemicals in treated water, different approaches can be added to the conventional water treatment process. Firstly, the secondary sedimentation stage can be expanded to reduce nutrients including nitrates and phosphates. Secondly, chemicals can be used to neutralize residual chemicals in treated water. Lastly, an adsorption step can be added which can entail passing treated water through materials with microporosity such as activated charcoal or activated aluminum to adsorb all chemical residues and remove them from water. The third option is the cheapest and requires the least capital investment and running costs.

References

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