Wednesday, March 28, 2007

DIY Water Treatment, Part III

This post is the last of three discussing possibilities for self-reliance in water treatment using charcoal filtration. The first installment provided a general primer on the downsides of chemical pesticide application in industrial agriculture. In the second installment I summarized the results of our investigation into the particular agrochemicals of concern in northern Thailand. In this post I give some background on charcoal as a filter medium and describe our design for a drinking water treatment system.

DIY Water Filtration
Part III: System design

The US EPA, the World Health Organization and several academic studies identify granular activated carbon (GAC) as the best available technology for the control of many agrochemicals and synthetic organic chemicals in drinking water. Although it is not possible to produce high-grade GAC without an industrial process, lower-grade charcoal-derived GAC is readily made in earthen kilns and may exhibit appreciable capacity for aqueous contaminant adsorption. Studies have shown low-grade char from the burning of wheat straw to be 31 – 36% efficient for adsorbing dissolved pesticides compared with industrial-grade GAC. Taking conservative estimates for the capacity of homemade charcoal-based GAC to adsorb dissolved organic carbon (DOC), our research and calculations suggest that a maximum of 9 kg (20 lbs) of pulverized charcoal is needed to purify drinking water for one person for one year. In-depth molecular-scale characterization of various homemade charcoal materials to refine this estimate may permit a reduction in quantity by as much as ten times.

Charcoal filtering

Charcoal consists of elemental carbon in its graphite configuration. Carbon has been used for water purification for centuries, possibly dating back as far as ancient Egypt and India. Carbon, in the form of graphite, exhibits an exceptionally high surface area per volume: one gram of industrially produced activated carbon may have a surface area of 400 – 1500 m2 (a football field is about 5000 m2). Non-polar organic molecules dissolved in water are strongly attracted to this surface and bind due to van der Waals (induced dipole) interactions. Carbon filters are employed in commercial home water treatment systems (to improve water taste, for example) as well as in large-scale municipal treatment facilities.

Scanning electron microscope images of GAC. The grain on the left is about 1 mm across. The right image shows a close up of the pore space.

Carbon filters are effective for removing chlorine, mercury, iodine, and some inorganic compounds as well as many problematic organic contaminants such as hydrogen sulphide (H2S), formaldehyde (HCOH), and volatile orgnanic compounds (VOCs). Activated carbon does not bind well to certain chemicals including lithium, alcohols, glycols, ammonia, strong acids and bases, metals, and most inorganic substances such as sodium, lead, iron, arsenic, nitrates and fluoride. As a general rule, carbon will bind non-polar materials while polar materials will tend to remain in aqueous solution. Most pesticides are organic and strongly non-polar and thus should display an affinity for adsorption onto the carbon surface.

Water contaminants that can be reduced to acceptable standards – according to EPA National Drinking Water Standards – by activated carbon filtration include: organic arsenic, chromium and mercury complexes as well as inorganic mercury, benzene, endrin, lindane, methoxychlor, 1,2-dichloroethane, 1,1-dichloroethylene, 1,1,1-trichloroethane, trihalomethanes, toxaphene, 2,4-D, 2,4,5-TP (Silvex), and p-dichlorobenzene.

Carbon filters have limited capacity for removing microbial contaminants and should not be considered a sufficient method for eliminating this risk. The World Health Organization recommends coupling charcoal treatment with chemical (e.g. iodine, chlorine) or UV disinfection to ensure removal of microbial pathogens.ii

Charcoal Making

Charcoal is made by pyrolizing wood or other organic matter such as coconut or rice husks, nut hulls, peat, etc. This involves heating the base material to temperatures of 600 – 900 oC in the absence of oxygen. “Activation” of charcoal typically refers to physical or chemical processes designed to increase the reactive surface area of the carbon. Industrial activation processes may use chemicals and/or steam to enhance surface area, although simply heating the material to sufficient temperatures can produce a significantly activated charcoal. In conjunction with researchers in the United States, we are currently planning experiments to determine the degree of activation of locally produced charcoals as compared with high-grade industrial materials.

Earthen kiln method

An earthen kiln has been constructed at Pun Pun farm using adobe bricks and cob (a mixture of mud and straw) for charcoal production. The kiln is roughly hemispherical, with an inner diameter of about one meter. The walls are made of adobe brick and plastered with cob to a final thickness of about ten inches. The heat intake is approximately cylindrical, about twenty-four inches long and eight inches in diameter. The lid is about eight inches thick and made of cob reinforced with wood, steel rebar, and medium-gauge wire mesh.

The earthen kiln at Pun Pun: the base under construction.

And the completed kiln base.

Photos showing kiln lid construction detail.

Cob was first applied beneath the frame. With the frame in place, cob was applied to the interior. Then the upper layer of wire mesh was nailed into place and cob applied over the top. We made efforts to commingle the layers of cob with each other as much as possible. Nails sticking out of the wood frame provided additional surfaces for cob attachment. A layer of sand-rich earth plaster completes the lid. Holes were drilled in the four handles protruding from the cob for connection to a rope and pulley system. Owing to the weight of the lid, we’re installing a block-and-tackle mechanism attached to the center support beam of the kiln building for raising and lowering the lid.

The chimney (yet to be installed) connects to the bottom of the kiln base and will be about three meters long. The chimney will be made of tin metal, jacketed by a flowing-water heat exchanger. Smoke from the charcoal making process will be condensed in the chimney and collected to make wood vinegar, a useful natural pest deterrent. Effluent water heated by the smoke will be used to supply a hot shower or stored for watering the nearby gardens.

We estimated that the kiln will provide perhaps as much as 50 kg of charcoal per batch. Once a sufficient quantity (mass) of charcoal is obtained, it must be pulverized into grains fine enough to pass through a 3 mm sieve. The grinding process is necessary to increase the material’s surface area and enhance contaminant adsorption. The plastic mesh bags that produce (e.g. potatoes) is sold in are widely available, are free or inexpensive, and make a good sieve for the pulverized carbon.

Plastic mesh bag sieve.

Other sources of charcoal

In addition to making charcoal on-site, a variety of charcoal types are available locally. For example, several varieties of charcoal made from mangrove and tropical hardwoods as well as bamboo are available at local markets. Also charcoal made from rice husks can be found throughout Southeast Asia as it is widely used as a potting soil matrix for starting plants. The potential of rice husk charcoal as a filter medium are especially attractive since the coals are small and thus require little or no grinding to attain sufficiently small grains.

Several earthen charcoal kilns are located along the main road just north of Mae Taeng. Here charcoal can be purchased for 2 - 10 Baht per kilogram ($0.03 – $0.08 per pound).

Earthen kilns near Mae Taeng

A pit kiln for clandestine charcoal making (using illegally logged wood) located in the national forest one mile east of the Panya community.

Other uses for charcoal and its by-products

Charcoal has many uses in addition to water treatment. It is the preferred cooking fuel for the majority of rural people who cook indoors over an open fire as it burns longer and hotter than the common alternatives (cornhusks, bamboo) and produces less smoke. Charcoal is pulverized and used as an additive to bar soap as a scrubbing aid and skin exfoliant. Very finely ground charcoal has medicinal qualities and is used to treat stomach and enteric infections, as well as poisonings and overdoses following oral ingestion (it prevents absorption of the poison by the gastrointestinal tract).

Wood vinegar (pyroligneous acid) is a by-product of the charcoal making process. It is distilled by passing the smoke through a long chimney or heat exchanger to encourage condensation of water and a mixture of volatile organic compounds driven off from the pyrolizing wood. Its principal components are acetic acid, methanol and acetone. It is reputed to be a natural aid for various uses including mild pain relief such as tooth aches and to sterilize and promote the healing of minor wounds. Wood vinegar is also a mild natural pest deterrent and can be applied to crops or to wood surfaces to protect from termites.

Wood vinegar collection from kiln chimney.

Water treatment system design

Although we do not (yet) have precise laboratory data regarding the capacity of our homemade and locally available charcoal to adsorb the agrochemical contaminants of concern identified in the previous post, it is possible to design a drinking water purification system around a general set of parameters and make conservative estimates regarding its capacity for contaminant adsorption. In this case, we regard dissolved organic carbon (DOC) as our general contaminant and design the system around its removal. Several studies have shown that DOC adsorption by granular carbon can block or displace other adsorbed organics such as pesticides. Therefore, we estimate the capacity of locally produced charcoal to adsorb DOC in general, assuming a modest propensity for adsorption on the part of the charcoal and a generous concentration of DOC in the local canal water supply.

We make the following assumptions:

5 L/person-day: drinking water needs (double the US-EPA recommendations, due to the hot tropical climate and physically demanding nature of the farm work performed by the communities)

30 persons: average annual population of the tri-community

10 mg DOC/g charcoal: adsorption capacity of locally produced charcoal

50 mg DOC/L: DOC concentration in natural surface waters


5 L/person-day x 30 persons = 150 L/day = 54,750 L/yr

50 mg DOC/L / 10 mg DOC/g charcoal = 5 g charcoal/L

54,750 L/yr x 5 g GAC/L = 274 kg charcoal/yr, or

9.125 kg charcoal per person per year (or about 20 lbs)

Note that this calculation admits no contribution of untreated harvested rainwater to the annual drinking water supply. Also, the assumption regarding the DOC adsorption capacity of locally produced charcoal is a conservative estimate based upon the reported capacity of char made from the open-air burning of wheat straw to adsorb particular organic pesticides. This material was shown to be 31 – 36% efficient for adsorbing dissolved pesticides compared with industrial-grade GAC.v Since the wheat straw is exposed to air during the charring process, it is likely that the charcoal purchased locally or made in the earthen kiln on-site is of appreciably higher quality than wheat straw char and thus may exhibit a significantly improved adsorption capacity, perhaps by as much as ten times. Also, it is likely that the DOC concentration of the local canal water is much less, perhaps ten times less than what was assumed for the above calculations.

All of these factors affect the outcome of the calculations strongly; thus taken together perhaps we may conservatively estimate that actual charcoal needs are one-tenth of the above reported value. However, additional experiments are recommended before serious revision of the estimates above can take place. To this end, we are sending samples of a variety of charcoals to a university laboratory in the U.S. for characterization. It would also be useful to analyze the local canal water for DOC content and the variability of this content over time throughout the year. While it is relatively easy to ship solid materials such as charcoal internationally, it is another thing entirely to ship liquids. We are attempting to make inroads with Thai universities or research stations to perform DOC analyses. Further literature search may also permit refining of our estimates.

Treatment system specifications

We recommend a three-stage treatment system: a sand pre-filter followed by a pulverized charcoal filter and germicidal UV lamp.

Conservative estimates suggest that the sand layer should be about 50 cm thick. A diffuser plate should be placed over the sand to reduce the turbulence of the influent and prevent channels forming through the sand.

Charcoal density is about 500 kg per cubic meter. Therefore, the volume of charcoal required to treat one year’s worth of drinking water for the tri-community is calculated

274 kg charcoal/yr / 500 kg/m^3 = 0.548 m^3

One concrete ring 50 cm high by 1 m diameter has a volume of (pi x r^2 x h) = 0.393 m^3. Thus the necessary volume of charcoal is roughly equal to 1.4 concrete rings in depth (0.548/0.393). The purpose of the gravel layer is to prevent clogging of the perforated pipe by carbon granules. This layer need only be about 20 cm in thickness.

Finally, we recommend running the filtered water under a germicidal UV light to eliminate bacterial and viral contaminants. We are currently gathering specifications of germicidal UV systems and will provide a feasibility report for this aspect of the treatment system separately. A combined system of charcoal filtration and UV decontamination represents, in our estimation, the best available technology to ensure a plentiful supply of clean drinking water to the tri-community. By employing this system, the tri-community would provide a demonstration of a working prototype for a simple, effective and inexpensive drinking water system extensible to communities worldwide seeking to advance their practice of self-reliant living.


Beth said...

you should hook up with my friend evan. he is working on a water pollutant/purification in letcher county, kentucky. (i visited bad branch falls and it is awesome.) it sounds like a lot of the stuff he is working on is pretty on par with what you are doing. his website and newsblog have a ton of info. it'd be sweet to hook up this information close to home.

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