BIOCYCLE L N T I ' R N A T I O N A L

Central America

Small-ScaleDigesters InCosta RicaThis study shows how small-scale digesters canprovide methane for household needs and improvefertilizer quality while reducing environmentaldegradation.

Stephanre Lonzmg, Rau/ Botero Botero and jay Martin

RESEARCH and development in diges-tion technology has focused on large-scale, capital-intensive systems, butthese systems are largely inaccessible tosmall-scale farmers, especially those indeveloping countries. Small-scale di-gesters are inexpensive and easy tobuild, which makes them an appropriatetechnology to enhance the environmentand livelihoods of farmers in the devel-

oping world. Our research team at theOhio State University is studying theability of small-scale digesters to treatagricultural wastewater and produceenergy, and exploring ways to increasethe efficiency of these systems. This re-search aims to provide small-scale farm-ers with digesters that produce methaneand electricity to meet household needswhile reducing disease and environ-

mental degradation caused by poorwastewater management.

Currently, digesters are concentratedin developing countries, with over fivemillion household digesters construct-ed in China and India alone. Digestersbuilt around the world vary in their de-sign complexity, construction materials,and costs. In developed countries, mostdigesters are concrete stirred reactors,in which a portion of the produced bio-gas is used to heat the digester. Many ofthe digesters located in developingcountries are plug-flow digesters, con-structed without heating or mixingcomponents. These digesters are adapt-able to any tropical climate and requireminimal maintenance.

Taiwanese-modt'l digesters are sim-ple, flow-through reactors consisting ofa double tubular polyethylene bag, PVCpiping, and vinyl hosing to transportthe biogas from the digester. The con-struction, materials, and labor costs of aTaiwanese-model digester can varyfrom $34 USD in Vietnam to $150 USDin Costa Rica. The digester can be con-structed to treat waste from one to 100pigs or cows and has an estimated life of20 years.

The wastewater flows through the 10-40 meter long tubular polyethylene bag,

Taiwanese-mode) digesters used on farms InCosta Rica treat waste from 30 cows usingeffluent to make compost ( I ) . Biogas frompigs supply fuel for 12 hout^ of cooking perday (2). Digester efHuent is transferred bygravity to two lagoons as a food source fortilapia and washing stalls (3).

48 BIOCYCLE FEBRUARY 2007

BIOCYCLE iNTEKNAilOXVL

TABLE I. Characteristics for each of the seven digesters studied

Farm

1234567

Elevation(m)

50505070

250350250

WasteSource

65 pigs30 cows40 pigs12 pigs7 pigs5 pigs3 pigs

WashesPer Day

2221232

WashingTime(Min)

45203020105| j

DigesterVolume

(m3)

706040404025

DigesterRetention

Time(Days)

n.i19.316.244.451.391.4

where methane-producing microorgan-isms convert the waste to biogas. The pro-duced biogas has a high methane contentand can be used directly as a cookingsource, eliminating the need to buypropane or collect firewood for cooking.In addition, wastewater treatment andpathogen destruction occur during the di-gestion process. The low cost of small-scale digesters and the value-added prod-ucts they produce result in an increase inhousehold income and a decrease in wa-ter pollution and deforestation.

Study Site

This study investigated seven Tai-wanese-model digesters located in theLimon Province of Costa Rica, CentralAmerica. Four of the digesters were atsmall-production farms and three di-gesters were at La Escuela de la Agri-cultura de la Regifin Tropical Hlimeda(EARTH University), an internationalundergraduate university specializingin the study of sustainable agriculture.The digester locations varied in eleva-tion from 50 m to 350m. All digestersused animal wastewater, with the ma-jority using swine manure. The di-gesters were identical in constructionmaterials, but differed in digesterlength, wastewater management,wastewater source, and retention times(Table 1). The manure for each digesterwas washed directly from the stalls intothe digesters. The frequency andamount of wastewater used in each di-gester varied, with an average stall-washing time of 15 minutes, two times aday. The retention time in each digesterranged from 11 to 91 days, dependingon the amount of wastewater used towash the stalls and the volume of the di-gester (Table 1).

Analytical Methods

Multiple inflow and outflow sampleswere collected from each of the seven di-

gesters during farm visits from July toOctober 2005. In total, over 200 watersamples were collected and analyzed forthe following: pH, temperature, conduc-tivity, biochemical oxygen demand(BOD), chemical oxygen demand(COD), turbidity, total suspended solids(TSS), ammonium (NH4), orthophos-phate (PO4), and total kjeldahl nitrogen(TN). An IR-30M methane meter and aZ-900 hydrogen sulfide meter (Environ-mental Sensors Co.) were used on-site todetect methane and hydrogen sulfideconcentrations.

Results and Discussion

This study revealed that low-tech,plug-flow digesters are able to producemethane and reduce wastewater contain-ments at levels comparable to high-tech,completely stirred ciigesters favored indeveloped countries. The Taiwanese-model digesters produced biogas withmethane concentrations greater than 60percent and reduced the organic matterand solids in the wastewater by an aver-

age of 85 percent (Table 2). Even withminimal internal control and highly vari-able influent water quality, these small-scale digesters produced high qualitybiogas and improved the fertilizer valueof the livestock wastewater.

The average methane concentrationof the digesters was 66 percent with thehighest concentrations at Farm 3 (73percent), which used swine manure andhad the liighest solids and organic mat-ter loading rates (167 mg TSS/L-dayand 221 mg COD/L-day, respectively).The lowest methane concentrationswere found at Farm 2 (62 percent),which used dairy manure and Farm 7(61 percent), which used swine manure,but had a low solids loading rate (40.6mg TSS/L-day). Hydrogen sulfide lev-els at all seven farms were below 0.01percent.

There were large decreases in solids(86 percent), organic matter (85 per-cent), and total nitrogen (46 percent)The total nitrogen concentration de-creased due to microbial uptake andsolids settling in the digester. Solids arenot removed from plug-flow digesters,which allow for a more complete diges-tion of the solids, but results ina high re-tention time for proper waste treatmentand methane production. If the solids inthe digester start to harden, the digesterbag is manually massaged to loosen thesolid mass.

Mineralization occurred during thedigestion process, increasing the aver-age ammonium concentration by 44 per-cent, which increases the usefulness of

TABLE 2. Average inflow and outflow wastewater data for seven Taiwanese-modeldigesters studied in Costa Rica

Temperature(Fahrenheit)

pH

Biochemical Oxygen Demand- BOD (mg/L)

Chemical Oxygen Demand- COD (mg/L)

Total Suspended SolidsTSS [mg/L)

OrthophosphatePO, (mg/L)

AmmoniumNH4 (mg/L)

Total NitrofjenTN (mg/L)

Average Inflow(Max., Min.)

79*F(26°C)(83°, 75=)

7.3(8.4, 6.8)

467(887, 1 56)

2970(4680, 957)

2210(3030, 769)

13.3(33, 3)

46.1(106, 14)

306(551, 160)

Average Outflow(Max., Min.)

79''F (26OO(82^ 77°)

6.6(7.1,6.2)

96.2(126,56)

472(802, 194)

319(964, 36)

15.4(30, 2)

82.2(149, 19)

166(228, 108)

PercentageDecrease/Increase

0Decrease

10Decrease

79Decrease

84Decrease

86Decrease

14Increase

44Increase

46Decrease

50 BIOCYCLE FEBRUARY 2007

LYfKRNATIONAL

the digester effluent as an organic fertilizer. The effluent fromFarm 6 would be a more suitable organic fertilizer due to thelijgh ammonium content (149 mg/L), while the effluent fromFarm 2 would be a less useful fertilizer (19 mg/L), but wouldhave less effect on aquatic life if discharged into nearby watersdue to its low organic matter and ammonium levels.

The pH did decrease from an average of 7.3 to 6.6 diu-ing di-gestion due to fatty acid production, but there were not anychemicals added to the digesters to keep thepHcircum-neutral.Farm 1 and Farm 3 had higher pH levels in the outflow (7.2 and6.8, respectively), as well as higher methane production (69 per-cent and 73 percent, respectively). The optimal pH is 6.4 to 7.6,with maximum methane production occurring above pH 7.These two farms also had high total nitrogen loading rates (286mg/L and 490 mg/L, respectively}, which could have led tohigher ammonification rates, thus increasing the pH levels ofwastewater and leading to a higher percentage of methane inthe biogas.

In order to run a generator fueled with biogas, the methaneconcentration needs to be greater than 55 percent and the hy-drogen sulfide concentration needs to be less than 1 percent. Allof the digesters in this study meet these minimum criteria topower a generator. The Taiwanese-model digesters in tbis studyhave the potential to be upgraded for electricity generationbased on biogas production and the percentage of methane pro-duced, but capital costs to purchase the generator and storagebags might negate the economic s'alue of the digester systemand complicate the management and operation of these systemsfor the average farmer.

Conclusions and Future StudiesThe digesters in this study were effective at producing a sus-

tainable energy source and improving the water quality by pro-viding a more useful organic fertilizer, reducing the impact ofthe wastewater on the receiving waters, and generatingmethane to meet tbe farmers' cooking needs. Orgaiiic matteri}nd solids concentrations were consistently reduced in the ef-Huent waters and ammonium concentrations were increased.The low-tech, plug-flow, Taiwanose-model digesters analyzedin this study were able to produce methane at levels compara-ble to high-tech digesters. The methane production in these di-gesters creates a number of indirect environmental and societalbenefits, including (1) a reduction in deforestation associatedwith firewood collection, (2) less hours devoted to firewoodcollection, (3) eliminating the need to purchase propane forcooking, and (4) a reduction in greenhouse gas emissions to theatmosphere.

Currently, there is a Taiwanese-model digester/generatorsystem at EARTH University in Costa Rica that is producing 4hours of electricity per day. Experimental studies are currentlybeing conducted by our research team to maximize digestion ef-ficiency in order to increase the electricity generation capabili-ties of these low-tech systems. We are determining what quan-tity of grease and fats can be added to Taiwanese-modeldigesters to maximize biogas production without requiring theaddition of buffering chemicals to keep the internal digestionenvironment at an optimal pH for methane production. •

Coauthor Stephanie Lansing is at the Ecological Engineering Pro-gram at The Ohio State University in Columbus, Ohio. She can becontacted at: lansing.10@osu.edu.

Seeing is Believing

^Matur i ty Index-/'Oxygen Depletion

ReleasekmmO

O L VITA*www.solvita.com

COMPOST FACILITYMANAGER

Veolia Water North America, the Nation's leader in thecontract operations, maintenance, and management ofcommercial, industrial, municipal treatment systems,has an immediate opening for the Facility Manager atits Baltimore City, MD Composting Facility.

The operations contract for this facility has one yearremaining with the potential for a longer term. Thisbiosolids facility produces a high quality compostproduct .sold in the tri state area. Responsibilitiesinclude managing the facility's safety and complianceprograms, daily operations, marketing and client rela-tions.

Ideal candidate will have up to ten years of facilitymanagement experience in a biosolids environment,performed technical work in process engineering, BSdegree in a Science or Hngineering> excellent comput-er, written, and verbal communication skills required.Some travel.

For consideration forward resume including salaryrequirements to: vwnaoscareers^^veoliawaterna.com

EEO/AA

BIOCVCLE FEBRUARY 2007 51