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17.3C: Purification of Drinking Water - Biology

17.3C: Purification of Drinking Water - Biology


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Water is purified with filters to remove larger protozoans, and by chemical or UV disinfection to kill bacteria and other small pathogens.

Learning Objectives

  • Illustrate the steps of drinking water purification

Key Points

  • Water is first passed through a system of filters and a coagulating agent is added to remove particulate matter.
  • Water is then passed through a membrane filter to remove large pathogens such as cryptosporidum and giardia.
  • To finalize the purification process, chemical disinfection (usually with chlorine or ozone ) or UV light is applied to the water to kill bacteria, viruses, and the hardy cysts produced by cryptosporidium and giardia.

Key Terms

  • ozone: A triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope (O2), breaking down with a half life of about half an hour in the lower atmosphere, to normal dioxygen. Ozone is formed from dioxygen by the action of ultraviolet light and also atmospheric electrical discharges, and is present in low concentrations throughout the Earth’s atmosphere. In total, ozone makes up only 0.6 parts per million of the atmosphere.
  • coagulation: The precipitation of suspended particles as they increase in size (by any of several physical or chemical processes).
  • protozoa: Protozoa are a diverse group of unicellular eukaryotic organisms, many of which are motile. Originally, protozoa had been defined as unicellular protists with animal-like behavior, e.g., movement. Protozoa were regarded as the partner group of protists to protophyta, which have plant-like behaviour, e.g., photosynthesis.

Drinking Water Purification

In order to purify drinking water from a source (such as a lake, river, reservoir or groundwater), the water must go through several steps to remove large particles and different types of pathogens.

  1. Pumping and Containment: Water is pumped from the source into holding tanks.
  2. Screening: Water is passed through a screen filter to remove large debris.
  3. Storage: Water is stored in reservoirs, tanks, and water towers in preparation for purification. Sometimes water is “pre-cholrinated” in this system to prevent bacterial growth while it is in storage.
  4. Coagulation and Sedimentation: Although there are many processes by which large particles are removed from drinking water, most water purification systems implement some kind of coagulation system. A chemical that causes particle aggregation is added to the water, and clumps of particles form and settle to the bottom of the reservoir. This is called sedimentation.
  5. Membrane Filtration: Membrane filters are able to remove all particles larger than 0.2 um. Larger pathogens such as giardia lamblia and cryptosporidium are trapped in these filters, but the cysts they produce are small enough to pass through.
  6. Disinfection: Before water is considered potable, it must be disinfected to remove any pathogens that passed through the membrane filter.

Methods of Disinfection

  • Chlorination is the most common form of disinfection. Chlorine is a strong oxidant, and rapidly kills many microorganisms, especially bacteria. Because chlorine is a toxic gas, it can also be dangerous to sanitation workers. Chlorine based compounds like choloramine are often used. Although chlorine is very effective against bacteria, it is not as effective against the cysts formed by protozoans (like giardia lamblia and cryptosporidium). Chlorine can sometimes leave residual byproducts in water.
  • Ozone is an unstable molecule that readily gives up one atom of oxygen providing a powerful oxidizing agent. This agent is toxic to most waterborne organisms. Ozone is widely used in Europe, and is an effective method to kill cysts formed by protozoans. It also works well against almost all other pathogens.
  • Ultraviolet Light is very effective at inactivating protozoan cysts, and will also kill bacteria and viruses. However, it is not as effective in cloudy water. It is sometimes used in concert with chlorination.

Quality of drinking water from the agricultural area treated with pitcher water filters

Background: Home methods of drinking water treatment through filtration have recently become quite popular.

Objective: The aim of the study was to compare chemical composition of unfiltered water with water filtered in households with pitcher water filters. Obtained results were discussed in view of the effect of analysed chemical components of water on human health.

Material and methods: Water samples were taken from water works supplies and from home dug wells from the agricultural area. Unfiltered water and water filtered through filters filled with active carbon and ion-exchanging resin and placed in a pitcher were analysed. Electrolytic conductivity, pH, hardness and the concentrations of calcium, magnesium, nitrate, phosphate and chloride ions were determined in water samples. Results of analyses were statistically processed.

Results: As a result of water filtration, the concentration of phosphates significantly increased and the concentrations of calcium, magnesium, electrolytic conductivity and pH decreased. No changes were noted in the concentration of chloride ions. Filtering water decreased the concentration of nitrates in dug wells samples.

Conclusions: Using water purification devices is justified in the case of water originating from home dug wells contaminated with nitrates when, at the same time, consumers’ diet is supplemented with calcium and magnesium. Filtration of water from water works supplies, controlled by sanitary inspection seems aimless.

Keywords: water filtration calcium magnesium nitrates phosphates chlorides acidity.


Water Purification : By Bioprocessing and Chlorine Disinfection

For maintaining purity of municipal/corporation water supplies its chlorination is highly effective in purification of water from microbial contamination thereby preventing spread of infectious diseases.

Not only municipal/corporation water supplies but also other water supplies must pass through a series of purification steps before pipe line supply to make it safe for either human consumption or for process industry and agriculture requirements.

In some cases, however, the water resource may be sufficiently pure to require only treatment with a disinfectant like chlorine, chlorocresol etc. depending on the quality of treatment desired and the purpose.

In many instances the condition of water resource is such as to require normally three stages of primary purifications namely, sedimentation, filtration followed by disinfection.

However, in recycling of used water in industries or for their disposal in natural water ways stringent treatment methods are needed to make waste water/liquid effluent safe for recycling, for aquatic life and prevention of water basin filling up by metallic or other solids. Reference is therefore made to water resources and the danger of their pollution.

(i) Oxic Bacterial Process Biotechnology:

It is well reorganized that there are limits to the self purifying ability of a natural body of water. This limit pertains to both capacity and the speed of self purification. In recent industrialized societies necessity of development of rapid man-made water purification has become obvious.

In a more recent process a mix of various strains of bacteria has been used to assimilate organic matter natural as well as man-made consuming oxygen and producing chiefly CO2 and H2O.

The technological success greatly depends on the type of water and a pollutants, their amount and concentration, the consistency of these determinants and last but not the least, on the speed of bacterial assimilation and the presence of inhibitory ingredients.

Many process biotechnology have been developed in laboratories and pilot plants. Their operational reliability is of utmost importance, both in order to ensure continuous supply and necessary quality of treated clean after the purification process. Oxic biological nitrification of organic present in polluted water is also one of the major concerns in this bioprocessing to avoid eutrophication.

(ii) Anoxic Bacterial Process Biotechnology:

Many microbes not only survive but even grow rapidly when starved of oxygen. For survival these microbes avail gene machines which encode enzymes essential for anoxic growth. The most successful bacterial groups can use nitrate as a substitute for oxygen/air.

In anoxic stage of water purification/treatment less energy is derived from nitrate reduction than from conventional cell respiration, so genes, for nitrate reduction are activated only when nitrate is present but oxygen is unavailable. However, the product of nitrate reduction is very toxic. Actually, nitrate respiring bacteria in water purification does make provision for removal of nitrate, so formed.

(iii) Integrated Oxic-Anoxic Process Biotechnology: Consideration:

In the water purification bacterial nitrogen metabolism is an important key to clean water. Bacterial metabolism of three molecular species: ammonia, nitrate and nitrogen, dominates the oxic-anoxic integrated biological nitrogen cycle. In oxic biotechnology nitrification plays major role while denitrification fixation governs anoxibiosis in the process biotechnology. A striking balance between oxic-anoxic bioprocessing and water pollution is important so that the objective of waste water purification is not defeated.

(iv) Oxic-Anoxic Design Scheme:

In order to overcome the problem of eutrophication in waste water purification nitrogen content in water must be removed otherwise, it will cause BOD increase. The removal of nitrogen from waste/water can be done by appropriate schemes of oxic-anoxic design systems for nitrification/ denitrification. Operational characteristics of three schemes for designs of nitrification and denitrification have been described below (Fig. 11.15).

This scheme depends upon the endogenous respiration of the activated sludge to achieve denitrification.

Here a portion of the influent waste water is by-passed to the denitrification tank to provide food for the facultative organisms thereby increasing the respiration rate and hence the denitrification rate. Scheme 3: It uses influent waste water which is nitrogen deficient as a food source for denitrifying organisms. The waste water should contain a readily available carbon source.

(v) Relative Advantages and Disadvantages of the Schemes:

In scheme 1 while the bioprocessing achieves a low nitrogen effluent, the slow rate of denitrification under endogenous respiration conditions results in a large denitrification tank. In scheme 2 while some reduces the required size of the denitrification tank it has the disadvantage of increasing the unoxidized nitrogen in the treated effluent and in most cases increasing the effluent BOD.

While scheme 2 practice will not contribute nitrogen to the effluent, careful controlled operation is required to avoid increasing the effluent BOD. Experimental results and experience indicated that economic use of this process scheme necessitates increasing the respiration rate and hence denitrification rate in denitrifying unit. It is also required for only carbonaceous BOD removal. If higher O2 level is maintained in the aeration tank for maximum nitrification rate, the power requirement will be about two and half times that required for conventional activated sludge process operation.

(vi) Acceleration of Denitrification:

Biodenitrification for removal of nitrate and nitrite could be accelerated by a using appropriate amount of methanol. Based on stoichiometric equations of biodenitrification it could be computed that 1 mole NO3 is equivalent to 5/6 mole methanol.

When the waste water which is to be treated contains dissolved oxygen (DO) it needs to be removed before denitrification step. This has been accomplished by adding extra amount of methanol. From these considerations the total amount of methanol (Cm) requirement could be computed by the following relation:

Here N0 is the initial nitrate, N is the initial nitrite and DO is the initial dissolved oxygen concentration.

2. Chlorine disinfection and biohazard:

Purification of drinking water by chlorine disinfection is an age old process. It is very effective in killing hazardous microorganisms in potable water. However, bio-molecular design engineering concerns of water chlorination hazard are being known in more recent years.

Over-chlorination in water is hazardous to human health. The relationship between the concentration of available chlorine and the time taken to kill the organisms in water is exponential i.e.

It has been stated that in human lymphocytes, chlorinated humic substances produced DNA strand cleavage but only at a concentration that caused cytotoxicity.


Biological Filtration: The Future Of Drinking Water Treatment?

Biological Drinking Water Treatment

For years, Canada's aboriginal communities struggled to deal with poor-quality drinking water.

Their cold, brackish groundwater was packed with high levels of calcium, arsenic, and a variety of other contaminants, and was nearly impossible to treat effectively. At one point in 2006, there were 86 First Nations in Canada under boil-water advisories, according to Health Canada.

Many had given up on the idea that these aboriginal communities would ever have truly safe, clean drinking water. As a last resort, a group of scientists decided to try an unconventional method that had struggled to gain acceptance for decades ­&mdash biological drinking water treatment.

&ldquoWe looked at a string of conventional treatments, and they didn&rsquot work,&rdquo says Hans Peterson, a microbiologist and the Safe Drinking Water Ambassador for the Safe Drinking Water Foundation in Canada. &ldquoBiological filtration has not been historically accepted, at least not in North America, but it showed promise and is up and coming.&rdquo

Peterson and his group designed a water treatment system that uses naturally occurring bacteria instead of chemicals to remove contaminants from water, and then treats the water a second time by reverse osmosis (RO). Conventional RO systems using chemical disinfectants were attempted in the First Nations previously, but the RO membranes had to be replaced after just one year due to chemical and microbial fouling. The RO membranes in Peterson&rsquos new integrated biological and reverse osmosis membrane (IBROM) treatment system could last up to 20 years.

&ldquoFor water that is as bad as this was, using RO is absolutely necessary,&rdquo says Peterson. &ldquoSo we designed a system that could optimize that process, and that had to be a biological treatment system.&rdquo

Today, 16 full-scale treatment plants in Canadian First Nations utilize Peterson&rsquos IBROM treatment system. Boil advisories have been lifted all over the country. Treating the poor quality water in Canada would have never been possible without the use of bacteria, Peterson says.

&ldquoBiological treatment is going to be the future of drinking water treatment,&rdquo he says.

Peterson isn&rsquot the only one who believes in the advantages of biological drinking water treatment, which has been growing in acceptance and popularity over the last five to ten years.

&ldquoMore and more utilities are implementing it, you are seeing it discussed at conferences, regulators are tuned into it and are starting to develop guidelines around it,&rdquo says Jess Brown, the chair of the Biological Drinking Water Treatment Committee for the American Water Works Association (AWWA). &ldquoPeople think of biological treatment as a wastewater process. But really it is a very natural and effective process.&rdquo

Compared to other drinking water treatment technologies that sequester contaminants and then remove them, biological treatment destroys contaminants entirety and is able to remove multiple contaminants at the same time. This cuts down on sludge production and bacterial regrowth. Biological treatment can be used to remove natural organic matter, color, chloroform, perchlorate, nitrate, nitrite, bromate, iron, manganese, selenate, chromate, arsenate, and a variety of other contaminants. It eliminates the need for chemical oxidation prior to filtration or settling, eliminates the need for chemical reduction methods, and produces innocuous end-products, thus reducing the risk of a contaminated concentrate stream.

&ldquoIn conventional treatment systems there is a constant battle between disinfection and disinfection byproducts (DBPs),&rdquo says Brown. &ldquoWith biological treatment there are minimal to no chemical additions required, so you remove disinfection byproduct precursors.&rdquo

Cutting back on chemicals can save a treatment facility a significant amount of money as well, says Peterson.

&ldquoWe have a surface water plant that used to use $15,000 worth of chemicals per month. Now, with the biological filtration system they only use $100 worth,&rdquo says Peterson. &ldquoWith a conventional treatment system the footprint is just too high and the water quality is too low.&rdquo

Implementing a biological drinking water system is fairly simple it is a basic &ldquoold school filtration system,&rdquo says Brown. But the relative rarity of the technology up until now does present some obstacles. A lack of education is the biggest challenge.

&ldquoThere are no current manuals of practice or guidance manuals,&rdquo says Brown. &ldquoSo when you start out, you can&rsquot go and get a manual and that feels less comfortable. But that will change.&rdquo

In Canada, Peterson and his team have worked diligently to get operators up to speed. An apprenticeship program has been created, as well as several training programs and guides.

&ldquoYou can&rsquot take a cookbook off the shelf and do this,&rdquo says Peterson. &ldquoWe are writing the cookbook as we speak.&rdquo

The AWWA is also working to educate the public. In March they hosted the 2013 Biological Treatment Symposium, which focused on the benefits of engineered and passive biological treatment systems from research and utility perspectives. Over 30 different technical sessions were offered.

As with any new technology, startup costs may also hinder some utilities. In large cities with a lot of water infrastructure, a dramatic change like moving to biological treatment would involve pilot testing, which could be expensive. Right now the technology is best for smaller, rural communities struggling with water quality, says Peterson.

Many academics and industry researchers have dedicated their time to making biological treatment more accessible and easier to understand. As manager of Carollo Engineers&rsquo Research Group, Brown leads the firm&rsquos biological drinking water treatment initiative. Other firms have similar initiatives.

&ldquoWe&rsquove really been digging into the nuts and bolts of this and we understand it better,&rdquo says Brown.

Brown feels that now, more than ever, is the time for biological drinking water treatment to come to the forefront. Costs of handling water treatment residuals are rising, there is a greater push for green technologies, more regulations limiting the formation of DBPs, and new contaminants that are particularly responsive to biological degradation are on the rise &mdash all factors that make biological treatment an effective solution.

&ldquoWe get calls weekly from people interested in biological drinking water treatment,&rdquo says Brown. &ldquoThere are still a lot of folks that are hesitant, but I think as we grow the number of plants out there that are using it, we will start to see it a lot more. There is a big, big push in the industry to go toward this.&rdquo