Sustainable Decentralized Water Treatment for Rural and Developing Communities Using Locally Generated Biochar Adsorbents
By Josh Kearns, MS
Synthetic chemical water contaminants: an often overlooked challenge in international sustainable community development
Contamination of drinking water sources by harmful synthetic organic compounds (SOCs), such as pesticides, is a major worldwide problem. Pesticide pollution appears twice in the top ten of The World’s Worst Toxic Pollution Problems Report 20111 by the Blacksmith Institute, and has been indicated in every year’s report since initial publication in 2006. Effective, affordable and scalable green treatment technologies for SOC removal that are accessible to communities in the developing world or in remote areas of developed countries are, however, lacking.
A recent review in Science(2) indicates that the 300 million tons of SOCs produced annually, including five million tons of pesticides, constitute a major impairment to water quality on a global scale. The report highlights particular challenges in developing countries, including the overuse of pesticides, prevalent ignorance of relevant environmental and health hazards and widespread unauthorized use of black-market chemicals. In Thailand, for example, 75 percent of the pesticides used are banned or heavily restricted in the West due to deleterious ecological and human health effects.3 The Science authors state that “small-scale, household-based removal techniques are often the only possible mitigation strategy due to the lack of a centralized infrastructure,” and call for the development of “reliable, affordable and simple systems that local inhabitants could use with little training.”
Unfortunately, SOCs are not yet ‘on the radar’ of major actors in the water-sanitation-hygiene (WASH) sector of international development.The UN Millennium Development Goals, for example, are only concerned with mitigation of biological agents of waterborne disease.4 I recently attended a major international conference on global water and health in developing communities.5 My presentation was the only one that considered SOCs in drinking water and presented a potential treatment technology.6 Microbial pathogens are often the most immediate threat to human health (e.g. diarrhea) and so focus on these disease agents is warranted. However, we cannot discount the threat of bio-accumulating chemical toxins, such as pesticides. The immediacy and scale of this problem is highlighted by, for example, a survey of Hmong tribe women living in Mae Sa Mai village, Chiang Mai Province, Thailand, that reported detection of DDT in 100 percent of mothers’ milk samples. A number of other biocides were also frequently detected, and infants’ exposure exceeded by up to 20 times the acceptable daily intakes as recommended by UN-FAO and WHO.7
Charcoal filtration has been used to treat drinking water for thousands of years,8 and is still widely practiced today, particularly in rural areas of the major charcoal-producing countries, such as Brazil, India, China, Thailand and throughout SE Asia.9 Locally managed charcoal filtration might represent the most effective barrier to SOC exposure available to households and communities in remote and impoverished regions of the world, as charcoal can exhibit properties similar to activated carbon.10 To date, however, no studies have quantified how effective charcoals are for water treatment.11
Summary and discussion of field and laboratory research outcomes
Regarding recent field studies and laboratory experiments investigating the potential effectiveness of traditional kiln charcoals and gasifier chars for water treatment, traditional kiln systems are used to produce charcoals from wood feedstocks typically for use as cooking fuel. Kilning processes are often highly polluting, energy-inefficient and time and labor intensive. Energy-efficient, clean-burning gasifier units that are typically used for cooking and space heating produce a residual char and are easier and more pleasant to operate and make use of a wider range of biomass feedstocks, including agricultural and forestry wastes and byproducts. More detail on the conceptual background of biomass gasification for char production is given at www.aqsolutions.org.
Charcoals produced from traditional kiln systems
Preliminary experiments show that some charcoals produced from traditional Asian village kilns (e.g., the 200-liter horizontal drum12 and brick-and-mud beehive models) exhibit appreciable adsorption capacity for herbicides. However, studies indicate wide variability in SOC uptake among charcoals produced by traditional technologies.13 Although these initial results are promising, traditional charcoal manufacturing systems are energy-inefficient and highly polluting, contributing substantial greenhouse gas emissions and often making use of unsustainable, and sometimes illegally, harvested feedstocks.14,15,16
When it comes to water treatment, not all traditional charcoals are created equal. We have monitored traditional charcoal production in 200-liter steel-drum/adobe kilns and brick-and-mud beehive kilns in collaboration with farmers and villagers in northern Thailand and the Thai Royal Forestry Department Wood Energy Research Centre in Saraburi Province. These observations provide simulations of the typical range of peak temperature and heating duration characteristic of traditional charcoal production systems using a programmable laboratory pyrolysis unit to generate experimental chars. Figure 3 indicates wide variability in herbicide uptake capacity of charcoals produced under a representative range of conditions. Charcoals exhibited essentially no uptake to ~ 80-percent removal under these experimental conditions. (Experimental methods and additional data are presented and discussed below.) Thus, the manufacturing conditions and resulting quality of the char product exert a strong influence on its potential effectiveness for water treatment.
Chars produced from biomass gasifiers
Energy efficient, environmentally sustainable and scalable production of consistent, highly sorptive chars can be accomplished with biomass gasification. Biomass gasifier stoves are rapidly being disseminated for household cooking in developing communities, as they provide energy-efficient combustion with reduced emissions17,18 and produce small batches of char from agricultural and forestry byproduct fuels during normal daily use.19,20 Intermediate and large-scale gasifier systems are also being deployed around the world for generation of biochar as an agricultural soil amendment to increase crop yields and sequester carbon.21,22,23 Gasifier char production is favorable from environmental and energy standpoints when compared with traditional charcoal manufacturing, since pyrolysis gases are combusted within the unit rather than emitted as pollutants,24,25 thereby providing the energy that drives pyrolysis and obviating the need for an external heat energy source. Also, biomass gasifiers can be readily coupled with other unit processes for biofuel collection and waste-heat utilization.26
Studies to date show that gasifier chars, particularly when operated in high-draft mode (for example, by augmenting airflow when necessary by a fan or blower) consistently develop enhanced physico-chemical characteristics, such as high surface area, microporosity and herbicide uptake capacity when compared with traditional kiln charcoals.31,32 Gasifier char may therefore be an optimal choice for sorption of pesticides, industrial and fuel compounds, human and livestock pharmaceuticals and other SOCs of increasing concern to water quality.
Figures 8 and 9 show N2 BET surface area (upper) and porosity (lower) of chars made 1) from split pine logs in a 200-liter, traditional-style steel drum and adobe kiln, 2) from uniform pine wood slats in a programmable laboratory pyrolyzer used to manufacture char under controlled temperature and atmospheric conditions, and 3) from a cookstove scale TLUD gasifier using pine pellets. (Surface area and porosimetry courtesy of David Rutherford, USGS.)
2,4-D (see Figure 10) was chosen as a test compound because of its environmental relevance as one of the most widely used herbicides worldwide, and one of the most commonly detected pesticides in environmental waters,33 as well as for its human health implications as a potential carcinogen and suspected endocrine disruptor.34 Its chemical properties also make it a challenging compound to remove by adsorption. Thus, if 2,4-D is taken up by a char, it is likely that most other pesticides would also be effectively removed.
Batch experiments used 100 mg/L of each char ground by mortar and pestle to pass a 200-mesh US Standard Sieve, and introduced to solutions initially containing 100 μg/L 2,4-D (US EPA MCL 70 μg/L; WHO Guideline 30 μg/L) and background organic matter at a total organic carbon concentration of 4 mg/L (to simulate natural waters). Experiment bottles were agitated for two weeks in order to reach equilibrium. The traditional kiln data are an average for three chars made from bamboo, split eucalyptus and pine logs charred in a 200-liter steel drum/adobe kiln. The lab pyrolyzer data displayed are an average of four chars made from bamboo, eucalyptus, longan and pine logs cut into slats of uniform size (15 cm x 10 cm x 1 cm) and pyrolyzed under controlled temperature and atmospheric conditions. The cookstove gasifier data displayed are an average from several batches of pine pellet char made in a one-gallon TLUD unit under natural draft (ND) and forced draft (FD, with an electric fan) conditions. The cookstove gasifier-FD and 55-gallon gasifier-ND chars removed 2,4-D below detection limits (2 μg/L), hence ~100 percent.
Current conclusions from laboratory and field research
In summary, compared with traditional charcoal production, gasifier char production is more energy efficient and emits far less atmospheric pollution. Furthermore, gasifiers can be operated with agricultural and forestry residues and byproducts and are ideally suited for small-grained, chipped or pelletized biomass fuels. Gasifiers can readily be linked with other processes and applications for capture and use of waste heat. Research has shown both small-scale (cookstove) and intermediate-scale (200-liter/55-gallon drum) pyrolyzers consistently achieve high temperatures (650 to 950°C/1,202 to 1,742°F) required for substantial development of surface area and porosity in the char product, concomitant with improved performance for herbicide uptake in batch experiments. Therefore, gasifier biochars are a promising, appropriate, low-cost and sustainable technology for affordable decentralized water treatment in rural and developing communities.
Furthermore, the use of biochar for water treatment does not preclude its eventual application as a beneficial agricultural soil amendment and carbon sequestration strategy. In fact, we recommend composting35 and soil application as the preferred mode of processing spent filter char. The best strategy for rural communities and small interest holders to utilize spent filter char is simply to allow ample time and favorable conditions for environmental microorganisms to biodegrade any sorbed contaminants. Elevated temperatures, such as those achieved during composting of organic wastes (for example in composting toilets), accelerate microbial activity and biodegradation processes. Moreover, based on recent research with carbon adsorbents, we do not expect significant contaminant release to soils and plants by leaching from spent filter char.36 A conservative approach to land application of spent filter char can be adopted, using low incorporation rates of ~ 100 kg of char per hectare.
Case studies integrating biochar filtration into multibarrier treatment trains for decentralized systems
Figure 11 illustrates a passive-flow, multibarrier, intermediate-scale (up to 3,000 L/day) water treatment system using a series of gravel, biologically active sand and char filters. The system costs less than $500 (USD, at local labor and materials costs) to construct and provides years of service with periodic maintenance of the bio-sand filter and char refurbishment every two to three years. The system was installed in February 2008 and serves a farm community in northern Thailand with a seasonally varying population of 10 to 100 people (average 40) and treats all water used on the farm for direct drinking, plus kitchen and restaurant uses (food and beverage preparation, washing dishes).
For households and communities in very remote areas, low-cost, decentralized water treatment for removal of biological and chemical contaminants can be accomplished using filter media generated/acquired locally. Figures 12 and 13 depict a passive-flow portable drinking water treatment plant that provides up to 300 L/day (enough to meet minimum daily drinking water requirements for 100 people) using a series of gravel, biologically active sand and char filters. Containment is provided by four 200-liter, BPA-free, high-density polyethylene (HDPE) drums. Empty drums weigh less than 10 kg (22.04 pounds) and can be carried into remote communities on foot, connected with a small number of PVC fittings and filled with media generated onsite and acquired nearby. The system costs about $125 to construct, can be assembled with minimal hand tools (e.g. a Leatherman multi-tool) and provides years of service, with periodic maintenance of the bio-sand filter and char refurbishment once per year. The system depicted in Figure 12 serves a boarding school for ethnic Karen migrant/refugee children on the Thai-Burma border.
Open-source instructional handbooks and videos detailing the design, construction and operation of these water treatment systems and low-cost gasifier units for production of enhanced water filter biochar from local surplus biomass can be accessed from the Aqueous Solutions website (www.aqsolutions.org).