Study On Aerobic Composting Of Bioleached Sludge At Commercial Scale And Its Mechanism

In China,the production of municipal sewage sludge has sharply increased with wastewater treatment capacity exceeding 157 million m3/d by 2014,which means an estimated 100,000 tons of conventional dewatered sludge with 80%of moisture content could be produced each day.Due to the absence of practical sludge treatment methods,most of the sludge were treated improperly and becoming new potential of pollution source.Composting is an effective process to realize the recycling and harmless utilization of sludge,and composting products have an extensive application prospect.However,conventional dewatered sludge,with 78%-85%of moisture content,is too wet to be composted alone unless a large amount of bulking agents or dried organic amendments are applied to reduce moisture content and increase air void,which leads to large operation areas to be required,high expense of bulking agents,difficult control of offensive odor,and low nutrient content in compost products.Thus,exploring a high-efficient dewatering technology to decrease sludge moisture content below 60%is crucial in solving these problems.Recently,sludge bioleaching with Acidithiobacillus species has been proved to be an effective process in removing toxic metals and improving advanced dewatering of sludge.The results obtained from the operation of engineering project that bioleached sludge could be dewatered mechanically into dewatered sludge cake with the moisture of 60%or below.Moreover,dewatered bioleached sludge maintains high content of organic matter.Composting of such semi-dried bioleached sludge can drastically reduce bulking agent dosage and save operating cost.However,composting of bioleached sludge has been rarely discussed in previous research.In view of the above,this paper studied the dynamic characteristics of bioleached dewatered sludge composting with windrow system and evaluate its compost product quality,and established a rapid method to assess the maturity of sludge compost product.The main observations and conclusions are as follows:1.The dewatered sludge samples were collected from wastewater treatment plants in Wuxi city,Jiangsu Province,and the total content of nutrient and heavy metals were investigated.Results showed that the average concerntration of organic matter,TN,TP and TK were 45.4%,3.49%,2.02%and 0.24%respectively,and the TN and TP contents were significantly higher than those in farmyard manure,which could meet the nutrient requirements to be applied as soil conditioner or in agricultural system.However,the concerntrations of sludge borne-metals exceeded over the standards in different levels,which suggested that high concentration of heavy meals was an essential restriction affecting sludge land application.2.Bioleaching experiment was conducted in a bioleaching reactor.Results showed that bioleached sludge could be dewatered into sludge cake with the moisture content of 55.8%.Meanwhile,no obvious reduction of sludge organic matter was observed while sludge-borne heavy metals(Cu,Zn,Cr,Ni)were partly removed from sludge during bioleaching process.In addition,the bio-availability of heavy metals in sludge could be lessened,which was helpful in subsequent land use of bioleached sludge.3.The dynamic characteristics of bioleached sludge composting and the quality of compost products were studied in an engineering scale composting facility.The amount of bulking agent required in bioleached sludge composting was only about 10%of conventional sludge composting.The pH value of bioleached sludge mixture was consistently 0.2-0.3 lower than that in conventional sludge composting system.Besides,organic matters in bioleached sludge were easier to be bio-degraded and the content of ammonium was obvious higher.Unexpectedly,the cumulative ammonia volatilization in bioleached sludge composting was only 51%of conventional sludge composting,indicating that bioleached sludge composting obviously reduced nitrogen loss.Compared with conventional sludge composting system,the relatively lower pH,the assimilation of nitrogen by microorganism and the presence of water soluble Fe in bioleached sludge composting system might jointly decide the decrease of ammonia volatilization,which contributes to higher fertilizer values and lower heavy metal levels exhibited in bioleached sludge compost product.4.After co-composting of bioleached sludge mixed with wheat bran,the maxima temperature as well as the highest organic matter decomposition rate were achieved,which suggested that the wheat bran was an efficient auxiliary material for bioleached sludge composting.The addition of mushroom residue reduced ammonia emission by 31%compared to wheat bran mixture,which resulted in 23.6%increase of total nitrogen than initial.Furthermore,mushroom residue was found to possess higher specific area and porous structure,which was considered as an excellent bulking agent by reducing nitrogen loss during sludge composting process.5.Ferrous sulphate,zeolite and calcium superphosphate were added separately into bioleached sludge composting to investigate their effect on ammonia emission.With the addition of ferrous sulphate and calcium superphosphate,amomonia volatilization were reduced by 94%and 30%,respectively.Additionally,ferrous sulphate has fixed the ammonium produced during sludge composting,which caused 45.6%reduction of nitrogen loss.The results showed ferrous sulphate was the highly efficient amendment during bioleached sludge composting.6.A rapid testing method was designed based on CO2 production to assess the maturity of bioleached sludge compost.By the study,CO2 production of completely mature compost should below 5000 mg CO2/kg vs/d under 48 h incubation.Based on the analysis of several compost samples made from different raw materials,this method was verified to be accurate and applicable which could be easily applied for composting engineering.7.The windrow composting system of bioleached sludge was designed and constructed.The process,engineering equipment,operational parameters and effectiveness of this composting system were introduced in details.Bioleached sludge composting system completely solved the problems often occured in conventional dewatered sludge composting system and the matured bioleached sludge composting products are suitable to be applied in gardens and parks,which would be of great social and economic benefits.The results of this study provide significant guidance for the engineering application of bioleached sludge composting.