Quantitative and Qualitative Assessment of Macronutrients and Micronutrients of Digestate from Biogas Units under different Feedstock Loadings

Digestate from biogas plants is a high quality product, suitable and safe for use as a fertilizer in agriculture. This study investigated the digestate produced from Anaerobic Digestion (AD) process through analyzing samples at specific times from digestate to estimate the nutrients concentration. The elements percentages of the macronutrients and micronutrients in digestate were evaluated, at different times for different feedstock. Eighteen typical biogas units each of 250 liters in volume were used to digest six combinations of feedstock (three replications of each feedstock type) for 50 days period. The feedstock combinations used were manure of cows, manure of sheep and goats, manure of poultry chicken, olive waste, combination of olive waste and cow manure and Kitchen residues. A daily feeding volume of 4.25 litters was mixed with water at a ratio of 1:1 and then entered to the biogas unit. The starting date of digestion for all biogas units was on, Sep. 12th 2013; ninety samples collected every 10 days. Using of digestate as fertilizer solves environmental problems of manure and organic wastes, and saves costs to farmers because they utilize their available resources. The digestate reduce manures and organic wastes pathogens and of weed seeds during AD process so this improves safety on farms. Using of digestate as a fertilizer is limited in Palestine due to the lack of information about its qualities and it is advantages. There was significant increase in the pH and macronutrients concentrations after digestion, this increase varied according to the type of feedstock. For the pH, the largest increase occurred in kitchen combination (6.57%) and the smallest increase occurred in cow combination (3.02%). For nitrogen, the highest percentage of nitrogen concentration increase was for poultry combination (13.88%) and the lowest for kitchen combination (11.10%). For phosphorous, the highest percentage of phosphorous concentration increase was for kitchen combination (16.8%) and the lowest was for olive combination (10.18%). For potassium, the highest percentage of potassium concentration increase was for cow combination (13.17%) and the lowest for olive combination (8.25%). There was a small increase ( not significant ) in magnesium, magnesium and chlorides concentrations. There was a little increase (not significant) of calcium concentration and a little difference of iron, manganese, zinc, copper and molybdenum concentrations during digestion. The best time to use digestate was found to be after 40 days .In average about 98% of changes of nutrients concentration occurred after 40 days. The changes of nutrients were from (65 to 80%) of total nutrients concentration after 30 days. Storage is required before digestate is applied to crops during the growing season. The most important issue in storage digestate is covering the storage because this protects nutrients from losses through ammonia emissions. Digestate must be applied during the growing season in order to ensure the optimum uptake of the plant nutrients and to prevent ground water from pollution. Digestate must be applied at certain amount and special equipment. Government support for farmers is required to establish biogas units and raise awareness among farmers to get the benefits of biogas and digestate. Agricultural researches are needed to clarify the needed nutrients for each type of soil and crop, the best combination of digestate for each crop type, the better method to applying digestate as fertilizer, monitoring crops growth during applying digestate and leakages of nitrogen. This research was done in autumn (September and October) when temperature ranged from 22 to 26 oC. If temperature gets more or less, as in summer and winter, many parameters will change according to temperature so research is required in other season of the year.