Possible Filter Technologies

We started our design process by researching all the possible technologies we could use to filter water. Below are the different technologies we researched with a synopsis on various parameters we considered including flow rate, operation & maintenance, and pro & cons.

Biosand

Contaminants it Removes : pathogens, turbidity, metals (uncertain about efficiency), 99% bacteria, 85-99% viruses, 99% protozoa

Operation & Maintenance : bio-layer takes a couple weeks to grow and must be maintained. Very easy to maintain because the top layer just has to be replaced.

Pros :

-cheap (can be built using local materials)

-can be made with concrete or plastic barrels, and sand

-removes pathogens and suspended solids from water

-helps reduce discoloration, odor, and unpleasant taste

-key to it being a “bio”-sand filter is the biological layer that “eats” bacteria, viruses, etc

Cons :

-takes a couple of weeks to develop the biolayer

-Biolayer must be “fed” every day

-Water freezes in Tibet, so biosand filter will not work

Ideas :

-prevent biological layer from dying of freezing temperatures by using the greenhouses TVP implemented to stave off malnutrition

Activated Carbon (A/C)

Contaminants it Removes : A/C can be used alone to improve tastes and odors, and it is most effective at removing organic compounds including VOCs, Radon, and Chlorine. It can also be used as pre-treatment for other water purification systems such as reverse osmosis and UV filters.

Flow Rate/time it takes to filter : The lower the flow rate, the more contact time the contaminant will have to diffuse into a pore and be absorbed. A/C will filter better with more time.

Operation & Maintenance : A/C bits cannot be reused and needs to be replaced.

Pros : can try to make own A/C by activating carbon found in a variety of resources, such as coconut shells, wood, or coal. Activation process is also quite simple and can be done with an industrial oven.

Cons :

-not available locally

-making own A/C is more work and a hassle

-need to replace often

Ideas: Good for taste and preliminary steps, maybe use as a step in a multi-step purification system. Could layer the A/C with a sari cloth in a ceramic container. A/C needs a lot of contact time in order to work properly, and the sari cloth would rid the water of particles.

Ceramic

Contaminants it Removes : bacteria, protozoa, microbial cysts, even viruses if we are able to add an iron oxide

Flow Rate/time it takes to filter : ~1-3 L/hour

Operation & Maintenance : filtering about 150L of water. Very low maintenance, just need to scrub clean when it gets too dirty or maybe just replace.

Pros :

-With the exception of the application of silver nitrate solution, all required materials are relatively cheap and widely available

-cheap to build and have been used with good success across Asia, Central & South America, and Africa

-this technology can be incorporated into the local economy quite well, especially if there are already potters in the village

Cons :

-not effective against chemical contaminants such as arsenic, fluoride, heavy metals, etc.

-if they crack, they become ineffective. Cracks are difficult to see as well so one can be unaware that there is a problem with the filter

-cracking is a big concern because of the cold temperatures in Tibet

-requires the use of liquid silver nitrate, which may be expensive and unavailable

-requires backwashing, making the silver nitrate solution require deionized water, requiring  precise measuring implements

-effectiveness depends a lot on quality control in building the ceramic pot

Slow sand (very similar to biosand)

Flow rate/time it takes to filter : ~30-40 L/day

Pros : lots of existing designs

Cons : impossible to maintain since don’t know how they would wash the sand

Fabric

Contaminants it Removes : particulates and some bacteria

Flow rate/time it takes to filter : depends on material; could be a few minutes to an hour or two

Operation & Maintenance : easy to backwash, just rinse in the river. Eventually will have to replace.

Pros :

-widely available and cheap

-many different types of fabric can be used (cotton and silk are most practical)

-easy to backwash

Cons :

-faster it filters, the lower quality of filtration

-community may not have silk fabric, which is the best kind because it’s tightly woven and will catch smaller particles

Membrane (nanofiltration)

Contaminants it Removes : reduces hardness, natural organic materials (NOM), micropollutants such as pesticides and VOCs, viruses and bacteria, salinity, nitrates, and arsenic

Operation & Maintenance : the major use of membrane in the US includes desalting, disinfection by products control, disinfection, clarification, and removal of synthetic and inorganic chemicals. NF(nanofiltration) and UF (ultrafiltration) need to be combined with conventional filtration techniques such as AC to maximize the efficiency and lower the cost.

Pros :

-depending on their use, membranes can be operated with minimal or no chemical pre-treatment that forms deleterious by products

Cons :

-techniques are partially mature (some lab experiments are being conducted on further development of the membrane techniques)

-hard to maintain

Natural Coagulants: S. potatorum seed or M. oleifera seed (moringa)

Contaminants it Removes : turbidity, heterotrophic bacterias, fecal coliforms, and viruses

Operation & Maintenance : A simple arrangement for home water treatment would be to dose 10 L of raw water  with S. potatorum seed or M. oleifera seed as coagulant, mix by hand-stirring for 2–3 min, and filter the water directly through a 500-mm sand filter. The sand filter may be made of a 1,200 mm long and150 mm IDPVC pipe, packed up to a depth of 500 mm with fine river sand, with 20–30 mm of pea-sized gravel at the bottom and arrangement for producing an initial flow rate of 3 L/min 21. Two to three filter runs may be conducted daily. When a considerable reduction in filtration rate occurs, the sand should be removed from the filter, washed clean with water and put back in the filter.

Pros :

-may not produce fecal coliform-free water but definitely produces water with ‘low risk’ according to the microbiological water quality classification scheme of the World Health Organization, based on thermotolerant (fecal) coliforms (WHO 1997)

-pH and temperature independent. will coagulate in possible ranges (pH of 6-10 and temps ranging from 50 to 100 degrees celcius

-in direct filtration natural coagulants exhibited bacterial removal rates that were similar to those achieved in a slow sand filter

Cons :

-needs sand filter too

-moringa needs warm temperatures to grow so cannot grow in Tibet

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