A study on pig wastewater treatment with low cost biodigesters

The performance of a 12.3 m³ of bag biodigestor for the treatment of swine waste was studied. Waste came from a small farm of 20 fattening pigs of Havana province.

Remove of total solids (TS), volatile solids (VS) and the chemical oxygen demand (COD) were respectively 74%, 69% and 71%. Temperatures of waste at entrance and exit were respectively 24.4 and 25 Celsius degrees, while environmental temperature was 24.7 Celsius degrees. Daily waste per day was 778 liters while water using for cleaning the barn was 694 liters total per day and 34.7 liters per animal per day. The use of water for cleaning was high, thus hydraulic retention time (HRT) was 15.9 days. Daily production of biogas (3.49 m³) was used for cooking food for a family of 6 people including a child. A 4 place kitchen stove was adapted for the use of biogas instead of the use of petroleum liquid gas.

Estudio del tratamiento de residuales porcinos con biodigestores de bajo costo

Resumen

Se realizó un estudio al comportamiento de un biodigestor de bolsa plástica de 12.3 m3 en el tratamiento del residual porcino proveniente de 20 cerdos en ceba en una pequeña granja en la Provincia Habana. El trabajo se efectuó durante los meses de Noviembre a Marzo.

La remoción de los sólidos totales (S.T), sólidos volátiles (S.V) y la demanda química de Oxígeno (D.Q.O) fue de 74%, 69% y 71% respectivamente. La temperatura en el residual a la entrada, salida del biodigestor y ambiente fue de 24.4, 25 y 24.7 grados centígrados. Se midió el gasto de agua para la limpieza que fue de 694 litros por día. El residual diario fue de 778 litros y la cantidad de agua de limpieza por animal y por día de 34.7 litros. La cantidad de agua utilizada en la limpieza fue relativamente elevada por lo que el tiempo de retención hidráulica ( TRH) fue de 15.9 días. El biogas producido diariamente 3.49 m³ se utilizó para elaborar los alimentos de la familia compuesta por 6 personas mayores y un niño pequeño; para este fin se adaptó una cocina de 4 hornillas y horno para utilizar biogas en sustitución del gas licuado.

Introduction

Nowadays the organic wastes treatment is a world problem and the 3^rd world countries are taking into account the different ways to treat them, to avoid the environmental pollution. The anaerobic digestion technology is a system which allows decreasing the waste pollution with the biogas production as ecological oil source, also. Therefore it is one of the alternatives to achieve this purpose with less environmental damage.

The production process of the Cuban swine enterprises takes place in two phases: the reproduction-weaning period and the growing-fattening period which take place in different housing. The waste treatment systems are very necessary to decrease the pollution in this kind of production system. The Swine Research Institute has been studying plug flow anaerobic biodigesters to treat the pig wastewater from the small and medium farms, similar to that used by Botero and Preston (1987). In this sense, the technology has been applied in rural zones successfully.

The present research studied the plug flow polyethylene biodigester as a system to treat the organic wastes from a small swine farm in Havana.

Materials and methods

A commercial plug flow polyethylene bag biodigester was used for the treatment of swine wastewater from a small farm with 20 fattening pigs (Figure 1).

Table 1. The main constructive parameters of the biodigester
Parameters	Unit	Amount
Total volume	m3	15.9
Digestion volume	m3	12.3
Gas storage volume	m3	3.58
Polyethylene bag diameter	m	1.59
Polyethylene bag length	m	8.00

Influent and effluent samples were taken. The analyzed parameters were: removal of total solids (TS), volatile solids (VS), the chemical oxygen demand (COD) and pH according to APHA (1981). The sampling was made three times per week from November to March, for 56 total samples. The results were analyzed by means of stadigrafics (Steel and Torrie 1980).

The cleaning water and the total production of installation wastes were measured. The biogas that was produced was used to cook the food of the family in the farm. The biogas production and the biogas used for cooking were measured by means of the installation of a gas meter counter (Dehm+Zinkeisen Dreieich model G4) made in Germany.

Table 2. Technical characteristics of the gas meter counter
	Unit	Amount
Minimum entrance of gas	m³/h	0.04
Maximum entrance of gas	m³/h	6.00
Gas volume	Dm³	2.00
Maximum pressure of gas	Bar	1.00

The gas meter counter was read daily. Temperature of the waste at the entrance and way out of the digester and the environment temperature were measured at 7:00 am and 5:00 p.m. To carry out these measurements a digital thermometer was used, model Anritsu made in Japan. The temperature results were processed according Harvey (1987).

Results and discussion

The total solids in the influent (2.77%) was low (Table 3), because the cleaning system to remove the swine waste required large quantities of water.

Table 3. Residual characteristics
	TS, %	VS, %	CO, mg/liter	pH
Digester entrance	2.77± 0.47¹	2.07±0.50	35520±5914	7.07±0.63
Digester exit	0.73±0.21	0.64±0.10	10458±1389	7.52±0.35
Removal, %	74	69	71	-
¹Mean and Standard deviation (n=56)

Similar values have been reported by Martinez et al (2005), with dilutions between 0.2 and 7%. Also, Vanotti et al (2002) reported values of total solids between 0.4 and 2.5%.

The removal rate of volatile solids (69 %) was higher than the 47% reported by Schulz and Miherleitner (1990). The COD removal rate in the biodigester is very important for a swine treatment system. These values are similar to those found in fixed dome biodigester evaluations (Chao et al 2000, 2005).

The pH of the effluent (7.52) is a typical value for anaerobic digestion systems (Esteban et al 1984; Hohlfeld y Sasse 1986; Fulford 1988).

Data on the animal population and quantities of waste cleaning water are in Table 4.

Table 4. Cleaning water, total wastes and hydraulic retention time
	Unit	Mean and SD#
Numbers of pigs	----------	20
Cleaning water	Liters/day	694±62
Total wastes	Liters/day	778±66
Cleaning water/animal	Liters/day	34.7±3.1
Hydraulic retention time	day	15.9±1.3
^# n=56

The quantity of cleaning water was high (694 liters/day). By using only one half the cleaning water/animal/day, it would be possible to treat the waste of twice the number of animals with the same digester or with a higher hydraulic retention time, which is better to achieve a less contaminated residual effluent.

The 34.7 liters/day/pig of cleaning water is higher than the levels reported (25 liters/day) by Vanotti et al (2002); however, it is lower than the levels reported by Juantorena et al (2000), which were from 60 to 80 liters/day/fattening pig of 100 kg of weight. The hydraulic retention time of 15.9 days is similar to that reported by Wellinger (2000), which was between 10 and 15 days to treat swine manure with 25°to 35° Celsius degree of temperature.

There were no significant differences between the temperature at the entrance and exit of the biodigester, and the environmental temperature (Table 5).

Table 5. Mean values for the temperatures at the entrance and exit of the biodigester, the environmental temperature and biogas production
	Unit	Mean and SD#
Entrance Temperature	⁰C	24.4±0.8
Way out Temperature	⁰C	25.0±0.6
Environmental Temperature	⁰C	24.7±1.6
Biogas Production	m³/day	3.49±1.22
^# n= 112

The daily biogas production was 0.28m³of biogas per m³ of digestion, similar to that reported by Marchaim (1992). Although this value is low according to the digester capacity, the daily production was enough to cook the food for six adult persons and a child under one year old. A 4-place kitchen store designed for petroleum liquid gas was adapted for the use of biogas instead.

Table 6. Economic parameters
	Unit	Amount
Total cost	USD	276
Cost-Benefits ratio	-------	1.81
Sensibility	%	81
Payback period	years	2.03
Net present values	USD	202
Internal Rate of Return	%	>50

It was considered a discount rate of 10 % and the project lasting time of 4 years. The use of this system gives an excellent economic effectiveness due to the results in cost-benefits ratio and the investment recovery period parameters.

According to the Costs Sensibility Analysis is possible to increase these results up to 81 %. Similar values were reported by Carballal (1997) in the economic study of PNUD-FAO 91/011 Project.

Conclusions

Bibliografia

APHA 1981 American Water Works Association and Water Pollution Control Federation. Standard Methods for than Examination of Water and Waste-Water, 15^th edn. American Public. Health Association, Washington D.C. 1134 p.

Esteban J, Gutiérrez J, Moré A, Ortega M y Sanz A 1984 Estudio del tiempo de retención (TR) y de sinergia de deyecciones de ganado en el proceso de fermentación anaerobia. Comunicaciones I.N.I.A. Serie Tecnológica Agraria II 104 p.

Fulford D 1988 Running a biogás programe: a handbook. Intermediate Technology Publications. London U. K., p 30-59

Harvey W H 1987 Mixed Model Least Squares and maximum likelihood program. User’s guide PC. 1 upp.

Hohlfeld J and L Sasse 1986 Production and utilization of biogás in rural areas of industrialized and developing countries. G.T.Z. Eschborn. F.R. G. p 51-96.

Juantorena A, Alfaro O y Sanchez I 2000 Alternativas para el tratamiento del Residual Porcino. Parte 1. Tecnología Química Volumen. XXI No. 2: 69-76

Schulz H and H Miherleitner 1990 Agricultural Biogás Plants and the use of slurry as Fertilizer in the Federal Republic of Germany. Report of International Conference on Biogás Technologies and Implementation Strategie, Pure/India. Borda, Bremen. F.R.G. p 541-561

Steel R G D y Torrie J H 1980 Principles and Procedures of Statistics: a Biometrical Approach. MacGraw-Hill Book Company Inc Toronto, pp 481

Vanotti M B, Rashash D M y Hunt P G 2002 Solid-liquid Separation of Flushed Swine Manure with Pam, Effect of Wastewater Strength. American Society of Agriculture Engineers Volume 45 (6): 1959-1969

Wellinger A 2000 Process Design of Agricultural Digesters. En Anaerobic Digesters Marking Energy and Solving Modern Waste Problems. AD-Nett Report 8-21 p.

A study on pig wastewater treatment with low cost biodigesters

R Chao, R Sosa, A A Pérez and E Cruz

Instituto de Investigaciones Porcinas,Gaveta Postal No.1, Punta Brava, La Lisa Ciudad Habana, Cuba

rchao@iip.co.cu

Abstract