Saeid Baroutian

A review of wet air oxidation and Thermal Hydrolysis technologies in sludge treatment.

With rapid world population growth and strict environmental regulations, increasingly large volumes of sludge are being produced in todays wastewater treatment plants (WWTP) with limited disposal routes. Sludge treatment has become an essential process in WWTP, representing 50% of operational costs. Sludge destruction and resource recovery technologies are therefore of great ongoing interest. Hydrothermal processing uses unique characteristics of water at elevated temperatures and pressures to deconstruct organic and inorganic components of sludge. It can be broadly categorized into wet oxidation (oxidative) and thermal hydrolysis (non-oxidative). While wet air oxidation (WAO) can be used for the final sludge destruction and also potentially producing industrially useful by-products such as acetic acid, thermal hydrolysis (TH) is mainly used as a pre-treatment method to improve the efficiency of anaerobic digestion. This paper reviews current hydrothermal technologies, roles of wet air oxidation and thermal hydrolysis in sludge treatment, and challenges faced by these technologies.

Formation and degradation of valuable intermediate products during wet oxidation of municipal sludge.

The current study investigated the formation of organic acids and alcohols as major intermediate products of wet oxidation of municipal sludge. Municipal sludge was subjected to 60-min wet oxidation at temperatures ranging from 220 to 240°C, with 20bar oxygen partial pressure. Acetic acid was the main intermediate compound produced in this study, followed by propionic, n-butyric, iso-butyric and pentanoic acids and methanol. It was found that the process severity has a significant influence on the formation and degradation of these intermediate products.

Phosphate recovery from hydrothermally treated sewage sludge using struvite precipitation

New technologies are needed to recover phosphate from organic wastes, such as sewage sludge. Sewage sludge can be hydrothermally treated to make it safe but this process is expensive. Recovering a valuable by-product, such as phosphate, could improve the economics of hydrothermal treatment. Therefore, the technical and preliminary differential cost analysis of combining hydrothermal treatment with phosphate recovery (by precipitation of magnesium ammonium phosphate (struvite)) was investigated. The effects of pH, magnesium ion dose, and either wet oxidation or thermal hydrolysis hydrothermal treatment were examined. Phosphate recovery was more sensitive to pH than magnesium ion concentration, with diminishing rates of recovery at high levels of both. Also, more struvite was recovered following wet oxidation treatment than thermal hydrolysis. Preliminary differential cost analysis showed that wet oxidation combined with precipitation at an optimal pH and magnesium ion dose could generate revenue.

A kinetic model of municipal sludge degradation during non-catalytic wet oxidation.

Wet oxidation is a successful process for the treatment of municipal sludge. In addition, the resulting effluent from wet oxidation is a useful carbon source for subsequent biological nutrient removal processes in wastewater treatment. Owing to limitations with current kinetic models, this study produced a kinetic model which predicts the concentrations of key intermediate components during wet oxidation. The model was regressed from lab-scale experiments and then subsequently validated using data from a wet oxidation pilot plant. The model was shown to be accurate in predicting the concentrations of each component, and produced good results when applied to a plant 500 times larger in size. A statistical study was undertaken to investigate the validity of the regressed model parameters. Finally the usefulness of the model was demonstrated by suggesting optimum operating conditions such that volatile fatty acids were maximised.

Hydrothermal processing of cellulose: A comparison between oxidative and non-oxidative processes

This study investigates oxidative and non-oxidative hydrothermal processing of cellulose at five different temperatures (180-260°C). Volatile fatty acids (VFAs) concentration, total suspended solid (TSS) degradation, dissolved organic carbon (DOC) and chemical oxygen demand (COD) were measured and compared in both processes. Moreover, the existence of hydrogen peroxide in both oxidative and non-oxidative processes was confirmed experimentally for the first time in literature. At temperatures ⩽220°C the amount of H2O2 produced in the oxidative process was higher (50 fold) than that of in the non-oxidative while at higher temperatures (⩾240°C) it was more for non-oxidative (3.5-5 fold). The concentration of VFAs in the non-oxidative process was lower than 10% of that in oxidative process. In both processes soluble COD increased with time and temperature, however at 260°C after reaching a maximum, it decreased with time due to conversion of some soluble intermediates to CO2 and water.

Variation in metals during wet oxidation of sewage sludge

Sewage sludge is a significant by-product from wastewater treatment plants but is potentially hazardous due to its pathogenic or harmful contents, such as toxic metals. Subcritical water wet oxidation (SCWO) is one method of hydrothermal processing which has recently been used in research and industry for treating sludge. This study investigated the effect of SCWO on metals such as Al, Cu, Fe, and Zn in sludge, including the content of metals in the liquid or solid phase after SCWO and their stability distributions according to Tessiers method. During SCWO, most metals were significantly accumulated and stabilised in the solid phase with very limited leaching. The temperature was found to be the most significant factor for metals immobilisation, followed by reaction time, while pH had negligible effect. The optimal conditions of SCWO on metals were found at 240°C, 60min, pH 5 in this study.

Application of hydrothermal treatment to affect the fermentability of Pinus radiata pulp mill effluent sludge.

A hybrid technique incorporating a wet oxidation stage and secondary fermentation step was used to process Pinus radiata pulp mill effluent sludge. The effect of hydrothermal oxidation at high temperature and pressure on the hydrolysis of constituents of the waste stream was studied. Biochemical acidogenic potential assays were conducted to assess acid production resulting from anaerobic hydrolysis of the wet oxidised hydrolysate under acidogenic conditions. Significant degradation of the lignin, hemicellulose, suspended solids, carbohydrates and extractives were observed with wet oxidation. In contrast, cellulose showed resistance to degradation under the experimental conditions. Extensive degradation of biologically inhibitory compounds by wet oxidation did not show a beneficial impact on the acidogenic or methanogenic potential compared to untreated samples.

Rheological characterisation of biologically treated and non-treated putrescible food waste

Food waste is gaining increasing attention worldwide due to growing concerns over its environmental and economic costs. Understanding the rheological behaviour of food waste is critical for effective processing so rheological measurements were carried out for different food waste compositions at 25, 35 and 45 °C. Food waste samples of various origins (carbohydrates, vegetables & fruits, and meat), anaerobically digested and diluted samples were used in this study. The results showed that food waste exhibits shear-thinning flow behaviour and viscosity of food waste is a function of temperature and composition. The composition of food waste affected the flow properties. Viscosity decreased at a given temperature as the proportion of carbohydrate increased. This may be due to the high water content of vegetable & fruits as the total solids fraction is likely to be a key controlling factor of the rheology. The Herschel-Bulkley model was used successfully to model food waste flow behaviour. Also, a higher strain was needed to break down the structure of the food waste as digestion time increased.

Physicochemical, structural and combustion characterization of food waste hydrochar obtained by hydrothermal carbonization

A solid carbon-rich product hydrochar, was prepared using hydrothermal carbonisation of food waste at temperatures of 200, 250 and 300 °C. To acquire detailed insight into physicochemical and structural properties, hydrochar samples were characterised using a range of techniques. The carbon content and higher heating value of food waste increased considerably from 39 to 73% and 15 to 31 MJ/kg corresponding to the heating temperature. The blends of hydrochar and coal prepared in three different ratios (5%, 10% and 15%) exhibited different thermal behaviour. The overall results of co-combustion study showed that the activation energy of hydrochar samples decreased from 56.78 KJ/mol to 29.80 KJ/mol with increase in temperature. Hydrochar prepared at 300 °C with coal blending ratio of 10% exhibited the lowest activation energy of 19.45 KJ/mol. Additionally thermal gravimetric analysis of the samples showed that high temperature carbonization can increase the combustion properties of hydrochar.

Energy performance evaluation of ultrasonic pretreatment of organic solid waste in a pilot-scale digester

It has been proven that ultrasonic pretreatment (UP) has positive effect on biogas generation from previous lab-scale studies. However, that is not always the case in larger scale processes. The purpose of this study was to evaluate the effectiveness of UP to biogas generation in terms of anaerobic digestion process and energy efficiency. Parameters including total solids (TS) and ultrasonic treatment operational parameters of organic solid waste (OSW) resulted from our past lab scale UP studies were applied in this study. OSW with 6-10% TS was treated using a lab-scale ultrasonic processor using various power densities (0.2-0.6 W/mL) at different time periods up to 30 min. Results of lab scale confirmed that OSW with 6% TS sonicated with 0.2 W/mL power density in 30 min gave the best outcome for the pilot scale experiment. To simulate the condition of an actual scale, in addition to energy analysis, two different organic loading rates (OLR), namely 500 and 1500 gVS/m3day were examined. The pilot digester was fed with OSW with or without the pretreatment based on the aforementioned specifications. The results showed that UP effectively improves biogas generation in terms of quantity and quality (CH4/CO2). Furthermore, it decreases the time to reach the maximum cumulative biogas volume comparing to the untreated feed. The key achievement of this research has confirmed that although the relative increase in the energy gain by the influence of UP was more remarkable under the 500 gVS/m3day OLR, energy analysis showed a better energy gain and energy benefit as well as jumping in biogas yield up to 80% for UP treated OSW under 1500 gVS/m3day OLR.