The Process And Characteristics Of Sludge Bio-Drying In High-Altitude And Severe-Cold Region

With the continuous improvement of China’s urban wastewater treatment capacity,the production of surplus sludge is also increasing sharply,and how to properly treat and dispose of sewage plant sludge is of great concern.With complex terrain,inconvenient traffic and fragile ecological environment in high-altitude and severe-cold region,sludge treatment and disposal and resource utilization are relatively lagging behind.In view of the fact that bio-drying technology has been applied in the treatment of wet organic matter such as cattle manure,municipal waste and other water evaporation or organic matter stabilization,and has the characteristics of fast water removal,no external heat source and low operating cost.In this study,we used bio-drying technology to dry and dewater the remaining sludge in high-altitude and severe-cold region,while a control test was set up in the middle and high altitude areas.The effects of the external environment,the intensity of induced air and the inoculation of functional microorganisms on the heating and water removal of the pile were systematically investigated.Investigate the process and characteristics of biological drying of wet organic matter and analyze the correlation between the community structure and succession of bacteria/fungi and environmental factors by combining the monitoring of basic indicators,organic matter metabolism and migration transformation,microbial population structure and enzyme activity.To develop a new technical approach for environmental protection and waste resource extraction in ecologically fragile areas of the Tibetan Plateau.For a high-altitude and severe-cold region,it can maintain the stable bio-drying process,and the heating and temperature maintenance of the pile are not significantly different from those in Kunming;only when the ambient temperature is extremely low does it affect the sludge bio-drying;higher ambient temperature（e.g.in the shed）helps the smooth bio-drying process.A small amount of induced air during the drying process is not conducive to the discharge of water from the pile,and a reactor with an airflow of 0.02 m³·h^-1·kg^-1 has a moisture removal rate of only 2.2%at the end of the bio-drying process,but a large airflow is not conducive to the maintenance of high temperatures.The inlet air volume required for bio-drying in high-altitude and severe-cold region is slightly higher,and the optimal induced air volume is 0.06～0.08 m³·h^-1·kg^-1.Affected by the low pressure and lack of oxygen and low relative humidity of air in the plateau area,the moisture contained in the sludge pile is more easily converted into steam and enters the gas phase in the upper part of the dryer mixture,and the relative humidity in the outlet gas increases significantly,which in turn gradually transfers the moisture in the wet material to the external environment.After 5-6 d of bio-drying treatment,the water content of sludge mixture decreased to 45%-48%in Lhasa and 50%-55%in Kunming.When biological drying is in progress,microorganisms use soluble organic substrate as substrate for aerobic metabolism,and the heat generated by them keeps the pile at a high temperature for a longer period of time,and the water in the wet sludge mixture evaporates continuously under high temperature conditions and is carried out of the drying unit with the airflow.The protein-like substances in the sludge pile are hydrolyzed into peptides and amino acid-like substances under the action of proteases,and then NH₄⁺is released under the action of microorganisms,i.e.the organic nitrogen in the pile is mineralized into inorganic nitrogen.At the same time,soluble inorganic salts are released from the pile.The continuous high temperature destroys the sludge extracellular polymer（EPS）and releases the intracellular material.When the biological drying is finished,proteins,sugars and inorganic salt ions are accumulated in the pile and soluble organic carbon（DOC）is converted from simple substances to compounds with a high degree of condensation.The C/N,EC,heavy metal content and calorific value of the pile meet the relevant standards for sludge treatment and disposal or utilization,and can be incinerated or land used.The intrinsic correlation between the metabolism of organic matter and water removal is that under aerobic conditions,the activity of microorganisms and enzymes is continuously enhanced,and during the respiration process of microorganisms,organic matter is dehydrogenated under the action of dehydrogenase,which leads to the synthesis of ATP resulting in the accumulation of energy,and the accumulated heat enters the water solid-liquid-gas three-phase cycle,and water vapor is taken out of the pile by way of air convection to ensure the smooth progress of bio-drying.Protein substances are hydrolyzed into amino substances such as peptides and amino acids under the action of protease,and then NH₄⁺is released under the action of microorganisms,organic nitrogen in the pile is mineralized into inorganic nitrogen.At the same time,soluble inorganic salts are released from the pile.The continuous high temperature destroys the sludge EPS to release intracellular substances,so when the biological drying is finished,proteins,sugars and inorganic salt ions are accumulated in the pile.DOC is transformed from simple substances to compounds with higher condensation,and the substances in the pile are based on a stable state.The C/N,EC,heavy metal content and calorific value of the pile meet the corresponding national standards and can be incinerated or land used.Inoculation of functional microorganisms can improve water removal from sludge piles,among which the best effect is Bacillus licheniformis,whose maximum temperature can reach 66.3℃,and the high temperature above 50℃can be maintained for 70 h.The moisture content can be reduced from 67.2%to 51.5%,the water removal rate is 15.7%and the degradation rate of organic matter is 13.3%.The variability of fungi in different sludge drying systems is more obvious.Unidentified was dominant in the early stage of sludge bio-drying treatment in high-altitude and severe-cold region,with its abundance up to 65.2%～71.1%;in contrast,unclassified＿Fungi（18.6%～56.6%）and Mortierella（24.8%,32.4%）were dominant in middle and high altitude.As the drying reaction proceeded,the succession of the dominant flora had similarity,mainly heat-tolerant and easily degradable organic matter or cellulose flora.Environmental factors such as pile temperature and organic matter content influenced the structure and succession of microbial populations in the sludge bio-drying process and were closely related to the degradation and migration transformation of organic matter.