Low-tech gasifiers for production of bone char as a low-cost filter media for fluoride reduction in contaminated groundwater Hobson, O. and Terrell, D. CATIS Mexico - November 26, 2015 - www.catis-mexico.org - dylan@catis-mexico.org
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Introduc)on Increasing +luoride concentrations in the Independence Watershed region in the state of Guanajuato, Mexico have become a pressing issue; with groundwater in some communities being tested at almost 16 times the 1.5 mg/L World Health Organization (WHO) and Mexican limits for +luoride. Levels higher than the WHO limit can lead to severe dental and crippling skeletal +luorosis in those who drink it and has been tied to cognitive and development issues as well. In order to address these problems, CATIS Mexico (CATIS) has been processing its own bone char - a type of biochar utilizing animal bones as the biomass - to ultimately be integrated into a lowcost water +iltration system. The bone char is produced using a Top-lit Updraft (TLUD) gasi+ier, designed speci+ically to pyrolyze animal bones. Bone char has been determined as the best, most economical adsorbent for +luoride for use in rural communities.
Background & History In 2014, Engineers Without Borders-UK (EWB-UK) researcher Will Mitchell began producing bone char at CATIS in controlled pyrolysis ‘burns,’ looking to see if animal bone type, burn temperature, and/or burn time had an impact on +luoride adsorption. Mitchell tested some of the bone chars on site but ultimately the majority of the samples were sent to Northern Illinois University (NIU) for testing. The results were promising, surpassing +luoride adsorption of standard bone char available on the market. The controlled bone chars produced by Mitchell achieved +luoride reductions between 96.3% and 99.8% in lab conditions (with the exception of one which achieved only 58.1% reduction). There was little noticeable difference seen between type of animal bones and burn time and temperature. When CATIS began constructing TLUD gasi+iers to produce the bone char, 100% +luoride reduction was achieved at NIU’s lab (highlighted in +igure 1 below). The promising results of the TLUD indicate that a low-tech bone char production can achieve a very high quality char that is perhaps better than high-tech operations. Figure 1 summarizes NIU’s testing of CATIS Mexico bone char. Figure 1: Northern Illinois Initial Testing
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The NIU tests were conducted in laboratory conditions, merely using distilled water spiked with +luoride. Groundwater in the Independence Watershed region is laden with other ions such as arsenic, which compete with +luoride for adsorption on bone char. Further, the water in the region tends to be much more alkaline, which has also been shown to reduce +luoride adsorption. The basis of the current research is two-fold: 1) to test various bone chars using in+luent water that exempli+ies typical regional groundwater, and 2) design an optimal and inexpensive gasi+ier - using local materials - for bone char production with the idea of making it easily deployable and operable in the +ield. All tests described in this report were performed using contaminated groundwater mixed from different communities, the initial +luoride concentration was determined throughout. All +luoride measurements were taken using a Hach +ield colorimeter using SPADNS2 as the +luoride reagent. Water testing and initial +ilter designs were performed by lead researcher, Olivia Hobson, Engineers Without Borders - UK, while working at CATIS Mexico. Gasi+ier design/construction and bone char production was led by Dylan Terrell, Executive Director of CATIS Mexico.
Current Work Ini)al Batch Tests Batch tests performed with CATIS bone char at NIU were found to obtain up to 100% +luoride removal; however, these results are unrealistic when using a contaminated groundwater. Tests were conducted using 40 g of char, placed in a ‘pouch’, in 1 liter of contaminated groundwater with an initial +luoride concentration of roughly 10 mg/L. Key +indings: • Minimum contact time of 24 hours • Minimum of 2 stirs throughout • 85% removal achieved • Batch method would not be appropriate for application at a community level • Fluoride has competition from other ions present in the groundwater; possibly hydroxide and arsenic
Figure 2: Batch test of bone char pouches
Ini)al Column Tests The initial column tests were performed in a 5 cm diameter PVC pipe packed with 75, 100 and 150 g of bone char. Key +indings:
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• • •
Smaller bone char particle size gives better adsorption (=< 2mm) Improved adsorption conditions than batch tests Need to improve bone char quality in order to increase +luoride adsorption as current bone char did not reduce +luoride levels to below the WHO limit
Following the initial column tests it was determined that the bone char quality needed to be improved in order to obtain better adsorption. Hence some modi+ications to the charring kiln were implemented in order to obtain longer burn times at temperatures between 400 and 550oC, with a lower, more uniform oxygen in+low and more fuel mass (wood).
Con)nued Column Tests Bone char quality Four different chars have so far been obtained through four different kiln designs (c.f. burn information for each char in excel spreadsheet). A piece of PVC pipe of 5 cm diameter was used for each column in which 100 g of each bone char was placed. Bone char particle size was equal to or less than 2 mm. The +lowrate only varied slightly between experiments and was at around 1 L/hr. The initial +luoride concentration of the feed groundwater was kept at around 10 ppm, apart from the experiment with Char 4 where the initial concentration was 7.3 ppm. The following graph compares the +luoride adsorption performance of each:
Comparison of different bone chars Residual fluoride concentration (ppm)
4
3 Char 1 Char 2 Char 3 Char 4 WHO limit
2
1
0 0.00
2.00
4.00
6.00
8.00
Amount of water treated (L) Figure 3: Comparison of different bone chars
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The graph shows that only Char 4 reduced the +luoride levels to below the required concentration, which suggests that more bone char will be needed for water with higher initial +luoride concentrations.
Filter ďŹ&#x201A;owrate Figure 3 shows that a lower +lowrate increases +luoride adsorption in the +ilter, which could be due to increased contact time. The residual +luoride concentration remains constant for a higher amount of liters treated at a lower +lowrate.
Effect of flowrate on fluoride adsorption Residual fluoride concentration (ppm)
6 4.5
11 L/hr 1 L/hr
3 1.5 0 0.00
2.00
4.00
6.00
8.00
Amount of water treated (L) Figure 4: Effect of flowrate on fluoride adsorption
Color issue An issue that arose during the testing was the discoloring of the bone char +iltered water. The yellow discoloring of the water (Figure 5) could be due to the presence of organic matter remaining on the char. Several measures were tested in order to address this coloring problem.
Sodium hydroxide wash
Figure 5: Yellow discoloring of the water after passing through bone char filter
Washing the char with sodium hydroxide (NaOH) has been
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suggested as a means of removing any leftover organic matter and so some bone char was washed with sodium hydroxide and tested in order to see the effect on color change. The +iltered water still had a slight coloring but was, on the whole, improved. However, due to this wash, +luoride removal was inef+icient (only up to 6% removal), which could be due to the increased presence of hydroxide ions, which compete with +luoride during adsorption. Sodium hydroxide can be used as a way of regenerating bone char, which could be researched further at a later date.
Hydrochloric acid wash Similarly, performing a hydrochloric acid (HCl) wash can remove any remaining organic matter. The processed bone char was washed in 3.6% hydrochloric acid bath over 24 hours and then washed with distilled water. The coloring of the +iltered water was signi+icantly improved with an
Figure 6: Coloring of filtered water through char washed with distilled (left) and through char washed with acid (right)
acid wash. However, the +luoride readings on the colorimeter were showing higher than the initial feed concentration, which could be due to an interference with the instrument of the chloride ions from HCl. Also, theoretically, a lower pH (that would be caused by the acid wash) should increase +luoride adsorption. These samples are being sent to NIU to determine if this is the case. This could be an applicable method for removing the coloring observed in the +iltered water, if the NIU results con+irm low +luoride levels.
Drying of the bones The bones previously used to obtain the char were dried in a solar drier for around 3-10 days. Having an increased drying time would reduce the organic matter on the bone before burning and hence, theoretically, avoid the discoloring. We were able to obtain some very dry bones without having to wait for the solar dryer. These were charred in the most recent kiln (c.f. burn
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info for char 4) and 100 g was placed in the same column as in previous t e s t s . T h e i n i t i a l + l u o r i d e concentration in the available groundwater was lower at 3.9 ppm. Having very dry bones before the Passive solar dryer built by CATIS Mexico
Residual Fluoride concentration (ppm)
Fluoride adsorption on very dry bones 4
3 very dry bones WHO +luoride limit Initial concentration
2
1
0 0.00
2.00
4.00
6.00
8.00
Amount of water treated (L) Figure 7: Fluoride adsorption on 100 g of char obtained from very dry bones at a filtration rate of 1L/hr
charring process proved to avoid the yellow/orange discoloring of the water (c.f. Figure 8). In addition, the +luoride concentration was reduced to below 1.5 ppm for 8 liters of treated water (c.f. Figure 7). Hence the drying time of the bones needs to drastically increase in order to obtain bones dry enough to char and hence remove all traces of organics and increase +luoride adsorption. After passing the water through the CATIS ceramic water +ilter, the water was taste tested and found to have a slight off-taste. A recommendation is to pass the bone char +iltered water through a biochar +ilter prior to the ceramic +ilter. Figure 8: Water obtained after passing through bone char and ceramic filter.
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Improving adsorp)on capacity Filter design and conďŹ gura>on A packed column has been shown to obtain higher +luoride removal than batch tests or bucket +ilters. The column is a piece of PVC and packed with bone char. According to the results of the tests carried out, a smaller bone char particle size increases adsorption. A particle size of 2mm or less was shown to have the best adsorption, which should be used in the +ilter. The outlet of the +ilter should be above the packed +ilter medium in order to avoid the bone char from drying up. An example con+iguration is as follows:
5 mm tube
Bone char +ilter
Bucket of dirty groundwater
CATIS ceramic +ilter
Figure 9: Possible filter configuration
In order to maintain a low +lowrate, a rubber stopper that was pierced with a thick needle was placed on the outer end of the 5 mm tube. A biochar +ilter could be placed after the bone char +ilter to remove any potential coloring, odor, or taste concerns. Figure 10: Filter configuration and rubber stopper for flow control
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The rubber stopper kept the +lowrate to around 2 L/hr and improved adsorption, reducing +luoride to below 1.5 mg/L from an initial concentration of 8.7 mg/L, with a quantity of 300 g of char in the column. The char used was obtained with the current gasi+ier design, which is illustrated in the following section.
Residual fluoride in treated water Residual fluoride concentration (ppm)
1.6
1.2
Char 6 WHO limit
0.8
0.4
0 0.00
0.75
1.50
2.25
3.00
Amount of water treated (L) Figure 11: Fluoride adsorption in filter with slow flow (2L/hr), 300 g of bone char and an initial fluoride concentration of 8.7 ppm
Gasifier Design CATIS has designed, built, and tested more than a dozen gasi+iers. The current bone char gasi+ier (Figure 12) is able to limit the amount of air+low into the chamber so good pyrolysis is achieved. Stable temperatures above 400°C seem be obtained in the bone chamber; however, these temperature readings are taken from inside the combustion chamber but outside of the bone retort chamber and thus might not be representative of the inside of the bone chamber temperature. For accurate readings, CATIS will be installing temperature probes through the combustion chamber and bone retort to take inner readings with thermocouples. Research suggests that the burn should reach a constant temperature between 400°C and 550°C for at least 2 - 2.5 hours in order to produce a high quality char. Temperatures should not exceed 600°C
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Figure 12: Current gasifier design
as that has shown to affect chemical composition of the bone char and have a negative impact on +luoride adsorption. Further, a uniform burn obtained via pyrolysis is vital for a char with a good +luoride adsorption capacity.
GasiďŹ er assembly Roughly 300 equally spaced small holes are drilled into the base of a 55-gallon drum (right), which will be the base of the fuel chamber. A piece of 4 inch duct work , roughly 2 feet in length, with a 90° elbow is placed on +lat ground or dug out in a trench with the elbow facing upwards (+igure 13), forming the primary air intake. Bricks or blocks are placed around the duct work, and the base of the fuel chamber is placed on top of the duct work. The elbow should be in the direct center of base of the fuel chamber. All of the open air around the base
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Figure 13: Primary air intake
Figure 14: Fuel chamber and sealed base
Figure 15: Extended fuel chamber
of the fuel chamber is sealed with mud to allow full control over the primary air intake (+igure 14). The top 2/3rds of another 55-gallon drum are cut and connected to the base of the fuel chamber (+igure 15). A normal 55-gallon drum cinch should connect two â&#x20AC;&#x153;topâ&#x20AC;? ends together tightly. This extended chamber increases the fuel capacity and burn time to achieve the desired 2.5 - 3 hours burn. The fuel chamber is then +illed with wood or other biomass.
Figure 16: Bone retort/combustion chamber platform rods and insulating metal skirt
Next, two steel square rods are placed across the top of the fuel chamber (+igure 16, left). These rods act as a platform support both the bone retort and the combustion chamber. Metal sheeting has been wrapped around the platform rods to close the gap created between the fuel and combustion chambers, limiting the secondary air and not allowing +lames from the combustion chamber to escape (+igure 16, right). This metal sheeting also acts as a semi-insulating mechanism for the bone chamber.
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The fuel chamber is then ignited and the bone retort is placed on the platform rods (+igure 17). Once combustion is underway, the combustion chamber is placed over the bone retort on the same platform rods and inside the metals skirt (if applicable). The combustion chamber is made of a full-size 55-gallon drum with no top. Eight tabs are cut in the bottom of the drum (which will be the top of the combustion chamber), and a 6 inch piece of duct work is bolted to the tabs, forming the chimney. Given that the combustion chamber is placed when combustion is already underway, it is important to be safe. Bolting two angle-iron rods on either side of the combustion chamber can act as handles and give more distance between those placing the combustion chamber and the +lames coming off the fuel chamber. Figure 17: Bone retort sitting on platform rods. Note: this photo is an early gasifier design where the metal skirt was not utilized and the fuel chamber had not been extended
Figure 18: Bone gasifier in operation.
This small-scale bone char gasi+ier utilizes inexpensive materials found anywhere in the world, requires no electricity, is easy to operate and maintain, and can be easily replicated â&#x20AC;&#x201C; thus lending itself to community adaptation and community-led operation. Temperatures inside the combustion chamber upwards of 500°C+ for up to 2.5 hours; however, these temperature readings need to be taken inside the bone char retort chamber itself. CATIS will begin doing this
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in the next trials. It is assumed that the bone retort should reach between 400°C and 550°C (max) for at least 2.5 hours and not exceeded 600°C as that has a negative impact on the chemical composition of the bone char.
Arsenic Removal Research suggests that arsenic competes with +luoride for adsorption on bone char. The groundwater from the rural communities in the independence watershed is also contaminated with arsenic. An arsenic test performed on the raw groundwater and the bone char +iltered water con+irmed a reduction in arsenic concentration of roughly 50%. Hence, in order to increase +luoride adsorption, it is recommended to remove the arsenic prior to the bone char +ilter. This can be achieved by adding an oxidizing agent. This will need further research and will be necessary to implement in several communities due to the increasing levels of arsenic being recorded in the region.
Future work In order to reach a stage of being able to set up a pilot +ilter in a chosen community, the current +ilter design should be tested, as a closed system, with the bone char obtained with the very dry bones. A biochar +ilter should be included in order to determine if it can neutralize the taste observed previously. The NIU results from the acid washed bone char should be analyzed to determine whether acid washing is a viable option for removing organic matter. Other methods for speeding up the drying process should also be researched. The current bones should be left in the solar dryer for longer periods of time - between 1 and 3 months - to remove as much organic matter as possible. Further, CATIS has already begun to cut the bones down to very small pieces to increase the effectiveness of the solar dryer. Once a pilot is set up it will be necessary to have continued testing of the +iltered water in order to con+irm the +ilter’s operational period before the bone char becomes saturated and exceeds the WHO limit and will hence be necessary to change it. After successful +ilter pilots, a full-scale production pilot should be implemented in a local community willing to produce and sell their own bone char to recharge bone char +ilter systems. This could be a great model microenterprise opportunity that could be replicated throughout the region and beyond.
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