Sustainable Bio-Fertilizer from Anaerobically Digested Animal Manure

Sustainable Bio-Fertilizer from Anaerobically Digested Animal Manure

Abstract

Our team acquired and characterized dairy manure digestate from an anaerobic digester located in Galt, CA. We found that the digestate contains many macro and micro nutrients essential for plant growth as well as beneficial microorganisms for soil. We studied the nutrient distribution among different size particle and liquid fractions. The results aided us in the design and optimization of separation methods to obtain the fractions with the most nitrogen, phosphorus, and potassium. We also compared dairy manure digestate with food waste digestate. We discovered different nutrient distribution profiles between the two types of digestate. For instance, more phosphorus and magnesium are contained in the finer solids of the dairy manure digestate than that of the food waste digestate. The distribution of nitrogen, calcium, potassium, and sodium, on the other hand, were similar between the two digestates. Based on results from our lab scale characterization and analysis, we designed, constructed, and demonstrated a small scale integrated system with treatment capacity of 300 gallons per day of raw digestate. The system produced solid and liquid fractions and consisted of a screw press and vibratory screen for coarse solid separation, followed by a membrane filtration unit for fine solid separation, and finally ambient air drying and a pellet mill for pelletization. The final pelletized products have a high packing density, useful for transportation, and exhibit slow release fertilizer properties. The nitrogen content of the pelletized products is close to 4%. Zeolite adsorption was investigated as a method for removing ammonia and other nutrients from the liquid fraction, followed by application of the nutrient rich zeolite as a potential soil amendment. However, we discovered that zeolite treatment resulted in an increase in the sodium content of the liquid above desired levels for irrigation water. Consequently, we also developed a novel strategy for reducing sodium desorption by pretreating the zeolite with calcium chloride prior to liquid treatment. 4 Our pelletized products were tested in a greenhouse trial growing Black Seeded Simpson lettuce. The growth period of lettuce is shorter than for tomatoes. Lettuce was investigated in lieu of tomatoes, as previously stated in the Phase I proposal because of the additional time needed for biofertilizer research. The production of lettuce in California is over two million tons annually; hence, its selection as a suitable substitute. The greenhouse trial showed that our pelletized products were able to meet the nutrients needs of lettuce. However, only half the yield was obtained as compared to the synthetic fertilizer treatment, possibly due to the lower bioavailability of nutrients in the pellets in the short lettuce growing period (Figure 1c). The organic nitrogen in the pellets needed more time to mineralize. Different strategies in processing and formulation of pellet production as well as in timing and amount of biofertilizer application need to be further investigated with different varieties of lettuce. Higher soil electrical conductivities were found with pelletized products than with synthetic fertilizer but the obtained values would not be considered problematic for healthy soil and plant growth. Lastly, an economic analysis showed the developed integrated system to have good potential for profitability when operated in large scale, with a payback period of less than 10 years for most of the scenarios investigated.

Type
Publication
Final Report to US Environmental Protection Agency