February 21, 2007 | General

Biogas Production From Animal Manure And Vegetable Wastes (Iran)

BioCycle February 2007, Vol. 48, No. 2, p. 52
In a pilot test, two types of biogas equipment were used and evaluated; results indicate success.
Zoleykha Najafi and Neemat Jaafarzadeh

TWO TYPES of anaerobic biogas production units – floating gas holder (Indian model) and fixed dome (Chinese model) were applied in pilot scales using a combination of manure and vegetable wastes as raw materials in the reactors for a period of six months in Ahwaz city, the center of Khuzestan province in Iran. Ahwaz has a semitropical climate. Several types of cows and buffalo live in most villages near the city, where their droppings can be used for biogas production. It has been assessed that more than 30 tons/day of raw manure are available, with enough vegetable wastes available due to huge vegetable production in the area.
Feedstocks were mixed and loaded into the biogas reactors. Samples of the mixed feedstocks and the effluent materials of the reactors were taken to the laboratory for testing. The following tests were conducted: humidity, pH, phosphorous, sulfur, nitrogen, electrical conductivity, total solids, total dissolvable solids, total nondissolvable solids and fecal coliform.
The pilot plants were operated for six months and the results showed that more than 70 percent of produced gas is methane. Results obtained revealed the Chinese reactor is more efficient than the Indian type. Comparing nitrogen and phosphorous contents of the reactor’s biosolids with chemical and nonstabilized manure (fresh animal or human waste) shows that sufficient amounts of nitrogen and phosphorous existed. During the winter season, operating the biogas reactor is possible and gas as a energy source can be obtained. Also, a good quality biofertilizer is produced.
Specifications of the materials used are indicated in Table 1. Upon lapse of two months as of the date of the systems commissioning, the first effluent fertilizer was separated as shown in Table 2. In terms of fertilizer outflow, the Indian reactor surpassed the Chinese reactor with more product.
Upon lapse of one month of the systems’ model construction, the reactors started producing gas in an average ratio of 23 cubic meters/week. The Chinese model produced more gas than the Indian one. The mixture of the gases in question were analyzed; using RKI, model eagle and ammonia and carbon dioxide gases and employing Gastec tube detectors, amounts of methane, hydrogen sulfide, carbon monoxide, sulfide dioxide, nitrogen dioxide and oxygen contents were determined.
Examining the results indicates an increase of methane gas produced during the period of operation. Upon stabilization of the system and regulation of environmental conditions, anaerobic bacteria have been enabled to produce methane as an intermediate substance through decomposition of the organic materials. On the other hand, due to the sulfur and nitrogen ingredients of animal droppings, production of hydrogen sulfide and ammonia may not be avoided. Of course, the data obtained from the produced biogas mixture analysis indicates that the amount of hydrogen sulfide gas has been gradually diminished and reached zero. The ammonia gas was detected only once and in an insignificant amount. The findings also indicate that near the end of the operation period, due to an increase of oxygen, anaerobic conditions in a part of the system were interrupted, causing a decrease in production of methane and an increase in CO2. The increase in oxygen was probably due to unsuitable conditions in the system from the aspect of preventing emergence and exchange of air during the operation, as well as the method of introducing the raw materials.
Examination of biogas production indicates that successful results were obtained. The value of biogas as fuel depends on the percentage of its methane content. The mean concentration of methane is more than 33 percent, which is equal to Indian experiences with the floating gas holder reactor. The methane gas content depends highly on the quality of the organic materials introduced into the digester. It also should be mentioned that even though the quality of the supplied materials were subject to change during reactor loading, sufficient amounts of carbon, nitrogen and phosphorus were available. Phosphorus and nitrogen content in the output were as follows: Chinese reactor – N=11.6%, P=6.7%; Indian reactor – N=11.9%, P=6.6%. (The effluent had an average moisture content of 93.2 percent.) Inasmuch as these fertilizers are very sanitary and economically are cost-effective, by using them the lost substances of the farmland may be restored in the long and even short terms.
Z. Najafi is a student at the Ahwaz Joundi Shapour medical sciences university in Ahwaz, Iran. N. Jaafarzadeh is an assistant professor at the university. Najafi can be contacted via e-mail at najafi2368@yahoo.com.

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