Household rainwater treatment options (Part 2)

By Water Research Commission

The selection of appropriate household water treatment (HWT) systems requires prior knowledge of the quality of the water, as well as how the different options of HWT systems work and their effectiveness against different contaminants.

The various HWT technologies remove different types of contaminants to different levels. Thus, understanding the local water quality and contaminants will influence the selection of appropriate HWT options.

The main focus of HWT is on removing biological pathogens, because they are capable of causing diseases more rapidly than chemical contaminants. Nevertheless, a number of HWT options can also remove chemicals and improve physical qualities of drinking water.

It is worth mentioning that before addressing methods of treating water at the household level, it is important to emphasise the need to prevent and/or reduce source contamination. For domestic rainwater harvesting (RWH) systems, that amounts to preventing contamination from the catchment area and the conveying system from entering the tank.

Furthermore, health gains can only be achieved if the HWT product is adopted and its use sustained. Treatment of harvested rainwater will also depend on the intended use of such water, thus harvested rainwater for non-potable uses such as gardening and vehicle washing will require less treatment compared to the one used for potable purposes such as drinking and dishwashing.

HWTs have proved to be effective on different types of water, including harvested rainwater. Therefore, by encouraging the use of HWT options, communities take responsibility for their own water security. Furthermore, if communities are empowered with the knowledge and tools to treat water at home, many microbial diseases will be eliminated. Low cost, safety, and efficiency in the removal of contaminants are the desirable qualities of home-based water treatment devices.

In South Africa, the most commonly used methods for treatment of household drinking water are boiling, cloth filtration, and chlorination. Each rainwater treatment system needs to be designed according to the surface from which the rain will be harvested.

Pre-storage treatment devices

Pre-storage treatment methods are required to keep sediment, leaves, and other debris from entering the RWH system. Leaf screens and gutter guards meet the minimal requirements for pre-filtration of small systems, although direct water filtration is preferred. All pre-filtration devices should be low-maintenance or maintenance-free.

The purpose of pre-filtration is to significantly cut down on maintenance by preventing organic build-up in the tank, thereby decreasing microbial food sources. If rainwater is not pre-filtered, a large amount of organic matter in the form of leaves and dirt can enter the storage tank. Aerobic bacteria begin to consume the organic matter and use up all the dissolved oxygen. This sets up anaerobic conditions that allow anaerobic bacteria to predominate, resulting in odour. Other benefits of pre-filtering rainwater are reduced sediment build-up at the bottom of the storage tank and less tank maintenance.

  • Leaf screens

Leaf screens are installed over either the gutter or downspout to separate leaves and other large debris from rooftop runoff. Leaf screens must be regularly cleaned to be effective; if not maintained, they can become clogged and prevent rainwater from flowing into the storage tanks. Built-up debris can also harbour bacteria and enhance their growth within gutters or downspouts.

  • Gutter guards or screens

Gutter screens can be installed on the gutter to filter debris before they enter the gutter. One advantage of gutter screening is the large filtering surface area, gutterwhich can reduce maintenance of the filter surface. Some higher-quality gutter screening is nearly self-cleaning and requires very little maintenance. The micro-mesh screen can filter debris in the 80–100-micron size, which is beneficial for potable or indoor fixture systems that require superior filtration.

Organic materials are the most common elements to negotiate in pre-storage harvesting treatment. Credit: Gutterbrush

  • Downspout filters (leaf catchers)

These filters are placed either at the top of the downspout where it meets the gutter, or somewhere along the length of the downspout. These filters generally only provide coarse filtration of 3175–1587 microns and should only be used for rain barrel systems. A few models can filter to 280 microns and make good pre-filters for irrigation systems and indoor non-potable uses.

  • First-flush diverters

First-flush diverters direct the initial pulse of rainfall away from the storage tank. While leaf screens effectively remove larger debris such as leaves, twigs, and blooms from harvested rainwater, first-flush diverters can be used to remove smaller contaminants such as dust, pollen, and bird and rodent faeces. Simple first-flush diverters require active management, by draining the first-flush water volume to a pervious area following each rainstorm. First-flush diverters may be the preferred pre-treatment method if the water is to be used for indoor purposes.

  • Insect screens

The inlet of the tank should incorporate a mesh cover and a strainer to keep leaves from entering the tank and to prevent access of mosquitoes and other insects. The overflow should also be covered with an insect-proof cover such as plastic insect mesh wired around the pipe.

Post-storage treatment devices

The fundamental difference between centralised water treatment works and HWT is not the underlying mechanism for treating the water, but the point where such treatment is implemented. While the former is a combination of treatment methods, the latter (HWT) tends to rely heavily on a single approach. These approaches comprise: sedimentation, filtration methods, and disinfection methods (to be covered in the next article of the series). Those methods are discussed further in the following sections of this document.

Post-storage treatment of the water is critical for both health of the users and maintenance of the system. The level of treatment will depend on the intended use of the water. Water used for irrigation does not require the same level of treatment as water used for potable indoor purposes. To maximise effectiveness, a multi-barrier approach where more than one method of treatment is used is recommended.

Post-storage treatment of the water is critical for both health of the users and maintenance of the system.

  • Sedimentation

Sedimentation is recommended as simple treatment of water prior to application of other purification treatments such as filtration and disinfection methods. It is a physical treatment process used to reduce the turbidity of the water. Small particulate suspended matters (sand, silt and clay) and some biological contaminants are removed from water under the influence of gravity. The longer the water is allowed to sediment, the more the suspended solids and pathogens will settle to the bottom of the container. The addition of special chemicals or some natural coagulants, such as indigenous plants, can accelerate sedimentation.

Four common chemicals used are aluminium sulphate (alum), polyaluminium chloride (PAC or liquid alum), alum potash and iron salts (ferric sulphate or ferric chloride). Some indigenous plants are also traditionally used in some countries, depending on the local availability, to help with sedimentation. In countries like Malawi, Sudan, Egypt, and Malaysia, the application of Moringa oleifera seeds extract in water coagulation and softening has received a lot of attention. Another natural plant coagulant known to clarify turbid surface water is Strychnos potatorum. This type of coagulant is reported to be used in countries like Southern and central parts of India, Sri Lanka, and Burma. Much of the suspended material can be removed by simply allowing the water to stand and settle for some time. This retention time (from one hour up to two days, the longer the better) is required to settle particles to the bottom. Storing water for at least one day will also promote the natural die-off of some bacteria.

Simple sedimentation is often effective in reducing water turbidity, but it is not consistently effective in reducing microbial contamination. However, most viruses, bacteria, and fine clay particles are too small to be settled out by simple gravity sedimentation. However, attachment of these smaller particles (bacteria and viruses) to suspended particles would result in the formation of flocs that can then settle to the bottom of the tank due to their increased mass. The addition of coagulants reduces the time required to settle out suspended solids and is very effective in removing fine particles.

  • Filtration methods

Filtration is commonly used to reduce turbidity and remove pathogens. It is a physical process that involves passing water through filter media. There are several types of filters; some are designed to grow a biological layer that kills or inactivates pathogens and improves the removal efficiency. Various types of filters are used by households around the world, including bio-sand filters, ceramic pot filters, ceramic candle filters, and membrane filters.

The purpose of pre-filtration is to significantly cut down on maintenance by preventing organic build-up in the tank.

  • Sand filters

Studies have reported that bio-sand filters (BSF) are capable of removing 81–100% bacteria and 99.98–100% protozoa from harvested rainwater. Reports indicate that treatment of harvested rainwater with BSF can reduce bacteria, viruses, and protozoa by up to 4-log reduction. Further reports show that turbidity can be reduced by 84% with BSF. Other studies have also reported on the removal of microorganisms and turbidity by iron oxide coated sand filters. Results showed that the coated filter medium was able to remove 99% of coliforms and 96% lead. Biosand filters, however, have been reported to have a limited virus removal efficiency.

  • Ceramic filters

filter potableCeramic water filtration systems generally consist of a porous ceramic membrane, a plastic or ceramic receptacle, and a plastic tap. Water is poured into the upper portion of the receptacle, or directly into the membrane, where gravity pulls it through the pores in the ceramic membrane and into the lower portion of the receptacle. Water is safely stored in the receptacle until it is accessed through the tap. There are two main types of ceramic filters, the candle filter and the pot filter, which differ in the shape and assemblage of the ceramic membrane:

A cartridge that is found in common water purifiers typically starts with a ceramic filter. Credit : Pinterest

  1. Ceramic pot filters. The pot filter system is simpler, and consists of a single concave membrane, which sits inside the rim of the receptacle. Several field trials carried out in different countries have found ceramic pot filters to be effective in reducing diarrhoea. A study carried out in three regions of Guatemala reported that 91% of the filtered water tested was free of faecal coliforms. In Nicaragua, water quality analysis was performed on 24 filters in seven communities. Of 15 homes that had E. coli in their water, eight (53%) tested negative for E. coli after filtration. In Cambodia, water quality tests were carried out after 1 000 ceramic filter pots were distributed, and results showed that after up to one year in use, 99% of the filters produced water falling into a ‘low-risk’ range of fewer than 10 E. coli per 100mℓ.
  1. Ceramic candle filters. Candle filter systems consist of an upper receptacle that sits above and is separated from the storage receptacle. Candle elements, which are cylindrical, hollow ceramic membranes, are attached to the barrier that divides the two receptacles. The only way in which water can flow into the lower receptacle is if it enters the candle elements, which is where filtration takes place. In a randomised, controlled trial conducted among 80 households, in one community during the six-month design life of the ceramic filter elements, faecal water contamination was consistently lower among intervention households than control households. Geometric mean themotolerant coliform (TTC) was 2.9/100mℓ versus 32.9/100mℓ, p<0.0001. Overall, 70.6% of samples from the intervention households met WHO guidelines for zero TTC/100mℓ compared to 31.8% for control households.
  • Nanofiltration (NF) membranes

Nanofiltration (NF) membranes are an effective technology to remove dissolved organic contaminants. This type of treatment offers an attractive approach to thatchroofmeeting multiple objectives of advanced water treatment, such as the removal of disinfection by product precursors, natural organic matter (NOM), endocrine disrupting chemicals, and pesticides. Disadvantages of using nanofilters include the decrease of permeate flux (membrane fouling), which is a major obstacle to the application of NF membranes to water treatment.

Roofing materials play a big role in the type of rainwater treatment required. Credit: ThatchroofInfo

Fouling worsens membrane performance and ultimately shortens membrane life, resulting in increased operational cost. Membrane filters applied as post treatment helps to remove pathogens and suspended solids. Advances in low-pressure-driven membrane technologies such as microfiltration (MF) and ultrafiltration (UF) have been used in water and wastewater treatment due to their high efficiency, ease of operation and small footprint. A study evaluated the efficiency of a polyvinyl (alcohol) (PVA) nanofiber membrane/activated carbon column, for the treatment of harvested rainwater.

Results indicated that total coliform counts in the unfiltered tank water samples collected from the two rainwater tanks had an average of 6×102 CFU/100mℓ. After filtration, total coliform numbers were reduced significantly (p=0.008) as a ≥99 % decrease was observed for all the first litres of filtered tank water samples in comparison to the unfiltered tank water samples. Furthermore, in separate experiments, molecular techniques were utilised to investigate the bacterial and viral removal efficiencies from RWH tanks.

Genus-specific PCR assays revealed the presence of potentially pathogenic bacteria, commonly associated with tank water. Results indicated that Klebsiella spp., Legionella spp., Pseudomonas spp., and Yersinia spp. were detected in all the unfiltered tank water samples and were then sporadically detected in the filtered tank water. Legionella spp. and Yersinia spp. were the most persistent genera, as these bacteria were detected in all the unfiltered tank water samples and in 85 and 80% of the 20 filtered tank water.

The PCR assays and BLAST analysis also confirmed the presence of bovine adenovirus 3 in all of the tank water samples collected before microfiltration for both tanks sampled. Other adenovirus strains detected in the rainwater tanks included simian adenovirus B isolate BaAdV-1 and human adenovirus 40 strain M-364. Moreover, once the tank water had undergone filtration through the PVA nanofiber membrane/activated carbon column, the presence of adenovirus was indicated in 75% of the filtered tank water samples. Even though the system was able to remove indicator organisms in an efficient manner, the removal of opportunistic bacteria such as Yersina and the removal of viruses were very poor.

  • Reverse osmosis

Reverse osmosis is a natural phenomenon in which water passes through a semipermeable barrier from a side with lower solute concentration to a higher solute concentration side. Water flow continues until chemical potential equilibrium of the solvent is established. At equilibrium, the pressure difference between the two sides of the membrane is equal to the osmotic pressure of the solution. To reverse the flow of water (solvent), a pressure difference greater than the osmotic pressure difference is applied; as a result, separation of water from the solution occurs as pure water flows from the high concentration side to the low concentration side.

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