Organic matter release in low temperature thermal treatment of biological sludge for reduction of excess sludge production

Exploration of browning reactions during alkaline thermal hydrolysis of sludge: Maillard reaction, caramelization and humic acid desorption

The browning reaction produces melanoidins, 5-hydroxymethylfurfural (HMF) and humic acids which influence subsequent anaerobic digestion and protein recovery. This paper systematically evaluates the variation of organics that make sludge browning with heating temperature and reaction time, the effect of browning organics on protein recovery and anaerobic digestion, and finally proposes a pathway for the occurrence of the Maillard reaction (MR) in the sludge environment. The results show that the browning of sludge hydrolysate is related to the comprehensive influence of the MR, caramelization and humic acid desorption. The increase of temperature (80 °C-150 °C) and pH (9-13) will promote the extent of browning of sludge hydrolysate, and the sludge browning reaction basically stabilizes at the reaction time of 1 h. Humic acid and melanoidin could co-precipitate with the protein, thereby reducing the purity of the recovered protein. The inhibition of anaerobic digestion starts when the melanoidin concentration is 8.01 mmol/L. The three-dimensional fluorescence, GC-MS and FT-IR analysis show that melanoidins have the same functional groups and fluorescence properties as humic acid does, and the humic acid in the supernatant of the sludge treated with ATH was not only converted at its adsorbed state, but also possibly generated by the reaction of the dissolved proteins with polysaccharides. Finally, LC-MS/MS was used to identify the intermediate products of the MR and the possible structural formula of melanoidin. This study further clarifies the browning reaction in hydrothermal sludge treatment and provides help for the accuracy of subsequent studies.

Enhanced deflocculation of dehydrated sludge by rhamnolipid treatment coupled with thermal hydrolysis

The effect of a biosurfactant, rhamnolipid (RL), coupled with a thermal treatment was investigated to determine its impact on improving the deflocculation of dehydrated sludge from wastewater treatment processes. Results showed that the RL treatment positively impacted sludge conditioning to weak acidity and hydrolyzed the carbohydrates and proteins released from the matrix of the extracellular polymeric substance. When RL was coupled with high temperature thermal treatments (65 and 95 °C), soluble chemical oxygen demand and chemical oxygen demand solubilization increased by 9.6-19.7 times and 13.4-29.3%, respectively. The RL treatment reduced antibiotic resistant bacteria by 5.4-98.4%, and antibiotic activity was further accelerated by high temperature thermal treatment. The combination of biosurfactant and thermal treatment can effectively deflocculate dehydrated sludge and should be considered an alternative technology for the sludge management process.

Integration of biological pre-treatment methods for increased energy recovery from paper and pulp biosludge

The paper and pulp industry (PPI) produces high quantities of solid and liquid discharge and is regarded as the most polluting industry in the world causing adverse effects to environments and human beings. Hence changes in the way PPI sludge and waste materials are treated is urgently required. Nearly, 10 million tons of waste is generated per year, however PPI waste is enriched with many organic chemicalscontaining a high percentage of lignin, cellulose, and hemicellulose which can be used as valuable raw materials for the production of bioenergy and value-added chemicals. Pretreatment of complex lignocellulosic materials of PPI waste is difficult because of the cellulose crystallinity and lignin barrier. At present most of this waste is recycled in a conventional treatment approach through biological and chemical processes, incurring high cost and low returns. Henceefficient pretreatment techniques are required by which complete conversion of PPI waste is possible. Therefore, the present chapter provides the scope of integration of pretreatment methods through which bioenergy recovery is possible during the PPI waste treatment. Detailed information is presented on the various pre-treatment techniques (chemical, mechanical, enzymatic and biological) in order to increase the efficiency of PPI waste treatment and energy recovery from PPI waste. Along with acid and alkali based efficient chemical treatment process, physical methods (i.e. shearing, high-pressure homogenization, etc.), biochemical techniques (whole cell-based and enzyme-based) and finally biological techniques (e.g. aerobic and anaerobic treatment) are discussed. During each of the treatment processes, scope of energy recovery and bottlenecks of the processes were elaborated. The review thus provides systemic insight into developing efficient pretreatment processes which could increase carbon recovery and treatment efficiency of PPI waste.

A comprehensive review on rheological studies of sludge from various sections of municipal wastewater treatment plants for enhancement of process performance

Large quantities of sludge is generated from different sections of a wastewater treatment plant operation. Sludge can be a solid, semisolid or liquid muddy residual material. Understanding the flow behaviour and rheological properties of sewage sludge at different sections of a wastewater treatment plant (WWTP) is important for the design of pumping system, mixing, hydrodynamics and mass transfer rates of various sludge treatment units, optimization of conditioning dose and for sustainable sludge management. The current article provides a comprehensive review on up to date literature information on rheological behaviour of raw primary sludge, excess activated sludge, thickened excess activated sludge, mixture of raw primary and thickened excess activated sludge (mixed sludge), digested sludge, and biosolid under the influence of different operating parameters and their impacts on process performance. The influences of various process parameters such as solid concentration, temperature, pH, floc particle size, primary to secondary sludge mixing ratio, aging and conditioning agent doses on the rheological behaviour of sludge from different treatment units of WWTPs are critically analysed here. Yield stress was reported to increase with increasing solid concentration for all types of sludge whereas viscosity showed a decreasing trend with decreasing total solid concentration and percentage of thickened excess activated sludge in the mixture. Temperature showed an inverse relationship with yield stress and viscosity. Viscosity was reported to be decreased with decrease in pH. The effect of various conditioning agents on the rheological behaviour of sludge are also discussed here. The applicability and practical significance of various rheological models such as Bingham, Power Law (Ostwald), Herschel-Bulkley, Casson, Sisko, Careau, and Cross models to experimental rheological characteristics of various sludges were presented here. The reported results on various rheological parameters such as shear stress, yield stress, flow index, infinite, zero-rate viscosity, and flow consistency index of different sludge types obtained from the best fitted model were also compiled here. Conclusions have been drawn from the literature reviewed and few suggestions for future research direction are proposed.

Evaluation of thermal hydrolysis efficiency of mechanically dewatered sewage sludge via rheological measurement

In this study, laboratory tests of both low temperature (60-90 °C) and high temperature (120-180 °C) thermal hydrolysis (LTHP and HTHP) were performed on mechanically dewatered high-solid sludges (at total solid of 14.2 wt% and 18.2 wt%) to evaluate the extent of organic solubilization through rheological measurements. The effects of treatment temperature and duration on organic solubilization and viscoelastic behavior of the sludge were comprehensively investigated. The results indicated that the organic solubilization contents including soluble chemical oxygen demand, soluble protein, and soluble polysaccharides increased logarithmically with the treatment time. Protein solubilized considerably faster than polysaccharides during thermal hydrolysis. The rheological curves exhibited the Payne effect in the amplitude sweep oscillation test. The elastic modulus in linear viscoelastic regime decreased logarithmically with treatment time. The viscoelastic behavior of sludge was well modeled by the Kaye-Bernstein-Kearsly-Zapas (KBKZ) model with paralleled Maxwell elements to describe the frequency dependence of elastic modulus and viscous modulus. With respect to the relaxation spectrum, the relaxation modulus first decreased with relaxation time and then increased. The relaxation modulus in each Maxwell element decreased with the treatment temperature and duration. Furthermore, in the HTHP, the influence of treatment temperature on enhancing organic solubilization and decreasing viscoelasticity exceeded the influence of treatment duration. In contrast, the treatment duration played a more important role than temperature in the LTHP. The content of organic matters was linearly related and logarithmically related to the elastic modulus in the LTHP and in the HTHP, respectively. The rheology analyses demonstrated that viscoelastic properties could be used as indicators to estimate the extent of organic matter solubilization in thermal hydrolysis process. The developed viscoelastic model provided insights for future research in numerically simulating the fluid dynamics of sludge.

Scientific basis of dissolved organic carbon limitation for landfilling of municipal treatment sludge--is it attainable and justifiable?

This study evaluated the scientific and technical basis of the dissolved organic carbon (DOC) limitation imposed on municipal sludge for landfilling, mainly for assessing the attainability of the implemented numerical level. For this purpose, related conceptual framework was analyzed, covering related sewage characteristics, soluble microbial products generation, and substrate solubilization and leakage due to hydrolysis. Soluble COD footprint was experimentally established for a selected treatment plant, including all the key steps in the sequence of wastewater treatment and sludge handling. Observed results were compared with reported DOCs in other treatment configurations. None of the leakage tests performed or considered in the study could even come close to the prescribed limitation. All observed results reflected 10-20 fold higher DOC levels than the numerical limit of 800 mg/kg (80 mg/L), providing conclusive evidence that the DOC limitation imposed on municipal treatment sludge for landfilling is not attainable, and therefore not justifiable on the basis of currently available technology.

Rheological characterisation of thermally-treated anaerobic digested sludge: impact of temperature and thermal history

This study investigated the partially irreversible effect of thermal treatment on the rheology of digested sludge when it was subjected to temperature change between 20 °C and 80 °C and then cooled down to 20 °C. The yield stress, infinite viscosity and liquor viscosity of sludge were measured at 20 °C for different thermal histories and were compared to the evolution of the solubilised chemical oxygen demand (COD) of sludge liquor. The results showed that thermal history irreversibly affects sludge rheology as the yield stress of sludge which was heated to 80 °C then cooled down to 20 °C was 68% lower than the initial yield stress at 20 °C. This decrease was due to the irreversible solubilisation of solid matter during heating as underlined by soluble COD data which did not reach its original level after thermal treatment. Measured soluble COD of sludge which was heated and cooled down was much higher than the soluble COD of initial sludge. We found a proportionality of the increase of soluble COD with the decrease of the yield stress as well as increase of infinite viscosity.

Comparing the effects of three pre-treatment disintegration techniques on aerobic sludge digestion: biodegradability enhancement and microbial community monitoring by PCR-DGGE

The objectives of this work were to compare and investigate the effect of three activated sludge disintegration processes before aerobic sludge digestion on 1) aerobic biodegradability enhancement and 2) microbial community evolution using the polymerase chain reaction-denaturant gel gradient electrophoresis (PCR-DGGE) technique. The comparison of three disintegration processes: thermal treatment (95 degrees C, 2h), sonication (100,000 kJ/kgTS) and ozonation (0.108 g O3/gTS) showed that the disintegration processes acted differently according to the composition of the soluble phase and to the DNA damage. Thermal treatment led to significant protein solubilization and to DNA modification. Sonication and ozonation resulted in similar soluble phase compositions and did not lead to any DNA modifications. During activated sludge aerobic digestion, intrinsic biodegradability enhancement was observed for thermal and ozone activated sludge pre-treatments. The analysis of the DGGE patterns at the end of aerobic digestion showed that population diversity was affected by both the aerobic digestion and the pre-treatment. The dissimilarity percentages measured at the end of aerobic digestion in the control sample and in the treated sludge were equal to 22, 25 and 20% for thermal treatment, sonication and ozonation respectively. This study indicated that PCR-DGGE could be a useful tool for the comparison of disintegration processes before and after aerobic digestion.

Monitoring of bacterial communities during low temperature thermal treatment of activated sludge combining DNA phylochip and respirometry techniques

Sludge reduction is one of the major challenges in biological wastewater treatment. One approach is to increase the sludge degradation yield together with the biodegradation kinetics. Among the various sludge pretreatment strategies proposed, thermal pretreatment at around 65 °C was described as promising. The enhancement in the biodegradation activity due to the selection of thermophilic hydrolytic bacteria was proposed, but further experiments are needed to demonstrate the specific role of these bacteria. In this study, concentrated activated sludge grown at 20 °C was subjected to thermal treatment at 65 °C for different periods. The originality of the work relied on a polyphasic approach based on the correlation between kinetics (chemical oxygen demand, COD; mixed liquor suspended solids, MLSS), bacterial activity (respirometry) and bacterial community structure (phylochip monitoring) in order to characterize the mechanisms involved in the thermal reduction of sludge. The bacterial activity in the aeration basin decreased to a very low level when recycling sludge was treated at 65 °C from 13 to 60 h, but then, started to increase after 60 h. In parallel to these fluctuations in activity, a drastic shift occurred in the bacterial community structure with the selection of thermophilic bacteria (mainly related to genera Paenibacillus and Bacillus), which are known for their specific hydrolases.

Techno-economic evaluation of thermal treatment, ozonation and sonication for the reduction of wastewater biomass volume before aerobic or anaerobic digestion

Aerobic and anaerobic digestions were compared with different sludge reduction processes such as ultrasonic, ozone, and thermal treatments. Each treatment was tested under the following conditions to improve batch aerobic or anaerobic digestion: ultrasound (200,000kJkgTS(0)(-1)), thermal (40 degrees C, 60 degrees C, 90 degrees C for 90 min, 120 degrees C 15 min, 1 bar), and ozonation (0.1gO(3)gTS(0)(-1)). The different pretreatments induced organic matter solubilisation and intrinsic sludge reduction (total suspended solids): ultrasound (47%), thermal 90 degrees C (16%), ozone (15%), thermal 60 degrees C (9%), thermal 40 degrees C (5%), autoclave (120 degrees C) (4.2%). TSS (and also VSS) solubilisation were found to be highly correlated to the pretreatment ability to break the flocs rather than to specific energy input. The total values of TSS reduction ranged from 57% to 71% under aerobic conditions and from 66% to 86% under anaerobic conditions. TSS solubilisation after pretreatment can be considered as a predictive parameter of sludge volume reduction enhancement after aerobic or anaerobic digestion while specific energy input did not show anything or negligible impact. In our experimental conditions, ultrasound and ozone led to the best TSS removal improvement after both aerobic (30% and 20%) and anaerobic digestion (20%). Ultrasonic and ozone pretreatments prior to aerobic or anaerobic digestion led to the best reduction of the specific energy required for removing 1 kg of TSS compared to the control. Anaerobic digestion was globally more effective (compare to aerobic digestion) in enhancing sludge production reduction.

Fate of cadmium in activated sludge after changing its physico-chemical properties by thermal treatment

The effect of thermal treatment of activated sludge on cadmium uptake was investigated in respect with potential modifications of floc surface properties. Soluble fraction biochemical composition and floc size measurements evidenced floc destructuration and the release of (in)organic ligands in solution. Characterization of sludge samples by potentiometric titrations and IR spectra showed the transfer of functional groups from particulate to soluble fraction as well as the higher availability of phosphate groups originating from cell membrane phospholipids after thermal treatment. Batch biosorption tests demonstrated that cadmium uptake was highly affected by sludge modifications due to thermal treatment. For temperatures below 95 degrees C, floc size decrease allowed a better availability of binding sites, resulting in a higher sorption capacity. At temperatures above 95 degrees C, the effect of released soluble ligands and of the lower total number of surface functional groups limited cadmium uptake. Uptake mechanisms were also affected by sludge thermal treatment as surface complexation involving ion exchange tends to become predominant over precipitation.