Study On Methane Production Of Biomass Waste Enhanced By Lime Mud From Paper-making Process

Food waste (FW) has high yield and is increasingly speeding up with rapideconomic development and urbanization process. Actually, FW is a pollutant, and a kindof renewable resource, simultaneously. Currently, anaerobic digestion (AD) is becomingone of important biological technologies adopted to dispose of FW at home and abroad,especially in our country in recent years. Due to the easy degradation of various organicmatters in FW, it is likely to result in acidification phenomenon, which is the mainreason to typically restrict anaerobic biogas degree and gasification production rate ofFW. Lime mud from paper-making process (LMP) mainly contains CaCO3, slightamounts of metal compounds, and trace inorganic elements with the potential for easingacidification process of FW, preventing excessive accumulation of organic acid, andavoiding "acidosis". Several kinds of wastes (e.g., FW, LMP and anaerobic sludge) wereco-digested to produce biogas (CH4/H2) in this paper, aiming at disposing of them forharmlessness, reduction and resourcization, widening the utilization approach of LMP,and providing theoretical and technical reference and support for the recycle utilizationof waste treated by another waste and energy-producing scale.Based on previous studies, the research of CH4and/or H2production enhanced byLMP through anaerobic single-phase batch reaction with the inoculation of anaerobicactivated sludge was conducted in this paper. It mainly included comparisons ofanaerobic methanogenic system performances synergistically enhanced by differentadditives, LMP dosage (concentration), FW pretreatment by heating alkali (LMP), andmesophilic and thermophilic conditions, respectively; effects of LMP on favoringthermophilic H2-producing; effect of acclimated sludge with LMP, and modified LMP(T-LMP) on thermophilic CH4-producing, etc.Main conclusions are as follows:Four additives (CaCO3, NaHCO3, LMP, and egg shell) could all improveanaerobic methanogenic system performance of FW and organics reduction. It wassimilar for these results of CaCO3and LMP test groups in terms of CH4-producingperformance, retention time (λ) and end time, etc. Compared to the control withCH4-producing rate of129.57mL/g VS and VS removal rate of18.5%, it wasincreased by86.5%and72.8%for LMP group with241.61mL/g VS, and VS removalrate of42.1%. This indicated that the process of AD from FW used LMP as an additive is feasible.Appropriately85-90%of CH4yield was mainly obtained in the former11days.Cumulative CH4production showed the "S" type, corresponding with the modifiedGompertz model. When the organic load was19.8g VS/L, the habitat of methanogenscould be improved at the initial LMP concentration of6.0-14.0g/L, with an significantincrease of both CH4yield (216.7-272.8mL/g VS) and VS removal rate (40.7%-47%),but an continuous decrease for λ (4.41-2.68d). The buffering capacity of a low initialLMP concentration (≤2.0g/L) was demonstrated lower, while high LMP (≥14.0g/L)inhibited methanogenic activity. An optimal LMP that favored the adaptability ofanaerobic microorganisms and disposal effect occurred at initial10.0g LMP/L with272.8mL/g VS and VS removal rate of47%.Compared with the direct effect of LMP on FW, it was increased by34.3-94.5%for CH4yield by facilitating its generation but had a longer λ (12.26-5.54d) frompretreated FW by alkali heated at80℃in water for1.0h when initial LMP ranged in6.0-14.0g LMP/L. However, experiments with low initial LMP (≤2.0g/L) wereinvestigated to cause "acidosis" due to high VFAs (4129.6-5214.2mg/L) and low pH(3.7-4.2). Actually,10.0g LMP/L had the optimal CH4-producing effect (448.5mL/gVS), which indicated that the FW pretreatment by heating alkaline (LMP) enhancedCH4production significantly, but not significantly for the metabolic process ofmethanogensH2yield and specific H2production rate were shown to increase dramaticallysince FW degradation and microbial growth could be accelerated by thermophilictemperature (55℃).Changes of specific H2production rate (6.0-14.34mL H2/g VS· h),λ (1.37-2.62h) and pH ranging from initial6.5-6.9to4.5-5.1were significant withLMP addition (0-13.3g/L). LMP addition (3.3-13.3g/L) to FW synergisticallyenhanced the H2fermentation capacity, and the maximum H2yield of137.6mL/g VSwas obtained at pH of5.0and10.0g LMP/L.When it was conducted at thermophilic temperature (55℃), parameters e.g., CH4yield, Rm, λ and so far changed linearly (rise/fall) as the initial LMP concentrationincreased. CH4yield (228-287.5mL/g VS), Rm (295.24-512.8mL/d) and VS removal(35.3-49.1%) were continually rising, while digestion time (25-11d) and λ (6.45-3.35d) were reducing within2.0-14.0g LMP/L, respectively. Meanwhile, high alkalinity(8771.3-10885.9mg/L) made anaerobic buffering system enhanced with pH range in7.5-8.0. Unlike mesophilic methanogenesis of LMP added to FW, the maximum effectwas achieved at the initial concentration of14.0g LMP/L with287.5mL/g VS and VS removal rate of49.1%.Sludge acclimated by1.0g LMP/L, and modified LMP (T-LMP) could acceleratemethanogens to adapt to the environment. The CH4-producing capacity of low T-LMPconcentration (0,2.0g/L) was better than those of high T-LMP concentrations (≥6.0g/L), but lower than that of the direct effect of LMP on FW, with CH4yield of128.8and135.2mL/g VS, and λ ranging from6.61d to3.15d, respectively. Additionally,high pH (10.0-11.8) of anaerobic system in groups (6.0-14.0g T-LMP/L) severelyinhibited the methanogenic activity, leading to the failure of start-up. It indicated thateffects of LMP-acclimated sludge and modified LMP on thermophilic methanogenesisof FW in AD were lower than that enhanced with LMP.Methanogenesis process is being conducted under multiple factors that worktogether and maintain a state of balance. Lack of metal elements, FW containsdegradable organics. Organic acids (e.g., VFAs, LCFAs, lactic acid, etc.) formed fromFW are likely to inhibit the growing metabolism of methanogens, and seriously,anaerobic reaction could even not be started. In the anaerobic CH4-producing system,HCO3-/CO2,and NH4+/NH3degradated from proteins/amino acids are the main internalfactors to regulate the acid-base equilibrium. However, buffering capacity formed fromlow concentration of ammonia nitrogen and a small amount of CO2was demonstratedlow in this paper. On the one hand, bicarbonate could be formed through neutralizingbetween organic acids and LMP to release the premise materials of CH4productionslowly, for improving buffering capacity. On the other hand, metal cations (e.g., Mg2+,Fe2+, S2-, Na+, K+) in LMP are required for microbial growth that promotemethanogenic metabolism. In addition, calcium stearate can be formed between Ca2+and LCFAs, which could reduce the inhibition effect of LCFAs on the utilization ofnutrients for microbes. The buffering capacity of modified LMP was testified to belower than that of LMP due to the exclusion of CaCO3. It indicates that exogenousLMP addition to FW has the role of enhancing its performance of anaerobic H2/CH4production synergistically.