Dr.I.M.mishraEnergyRecoveryfromIndustrialEffluents

ENERGY RECOVERY FROM INDUSTRIAL EFFLUENTS

Dr. I. M. MishraProfessor, Department of Chemical Engineering

Indian Institute of Technology, RoorkeeRoorkee –

247667, Indiaemail: [email protected]

ENERGY FROM INDUSTRIAL WASTES & BIOMASS

IIT Roorkee

is actively pursuing energy conversion through biomass-biochemical and thermal processing.

Different agri-forestry residues, grasses, leaves & stems, bagasseDifferent agri-industrial effluents

Thermolysis, acidogeneis, biomethanation.Pyrolysis and gasification

Slow & fast pyrolysisGas, bio-oil, charPongania pinata, Jatropha Curcas, Castor seed shell, Non-edible oil seed cake, Pongania pinata, Jatropha & castor seed cake

BiodieselSolvents – Ethanol, isopropanol, butanol using enzymes, microbial cells

ENERGY FROM INDUSTRIAL WASTES & BIOMASS

Recovery of energy from industrial wastewaters /wastesSugar mill press mudDistillery spent wash / sludge, bio-digestoreffluentPulp & paper mill wastewatersTextile mill wastewaters

Alcohol Distillery Spent Wash

The spent wash has a very high organic content (COD = 60-200 kg/m3) and is very complex in nature and colouredEffluent generation – 7 to 50 m3/ kilo litre of alcoholTreatment of this spent wash is a major pollution problem faced by distilleries (Chaudhary et al. 2005)

Spent wash contains reduced sugars, carbohydrates, waxes, proteins, alcohol, sulfurous compounds, minerals, melanoidins, etc.Biochemical route: Single / two-step biomethanation– aeration / aerobic treatment – ferti-irigation / composting with pressmud / sludge / dilution and discharge / reverse osmosis for recycle / reuse.

Typical composition of distillery wastewater

Typical Composition of the Biodigester Effluent before and after Catalytic Thermal Treatment (at T = 140 °C, pH0 = 1, Cw = 3 kgm-3, and tR = 6 h)

Typical composition of biodigester

effluent and the flocculated-treated effluent

Alcohol Distillery Spent Wash

Thermal route : Evaporation / concentration / incineration and ash disposal

Biomethanation – Thermo-chemical precipitation – wet-oxidation, sedimentation / filteration / reuse of water.

Elelctrocoagulation, membrane separation, sludge – slurry incineration.

Pulp and Paper Mill Wastewaters

Effluent from the pulping mill generated after cooking of wood or other suitable raw material is termed as black liquor due to its color.

It is highly organic in nature and contains organic matter in the form of suspended solids, colloids, BOD, COD, sulfur compounds, pulping chemicals used, organic acids, chlorinated lignins, resin acids, phenolics, unsaturated fatty acids, terpenes, etc.

The conventional treatment method of black liquor involves evaporation followed by incineration in furnaces.

Characteristics of pulp and paper mill effluent

Pulp and Paper Mill Wastewaters

Wastewaters from other sections of pulp-paper mills contain dissolved solids, all the above organic and inorganic chemicals, chlorinated compounds, phenols / phenolic compounds – most of them bio-refractory and color due basically to lignins and other refractory compoundsPulp and paper mill effluents contain 1 to 8 kg/m3 of COD and generate more than 150 m3 of wastewater per tonne of paper produced.Biochemical route reduces the BOD, but COD and colour are still insurmountable.

THERMO-CHEMICAL PRECIPITATION (THERMOLYSIS)

Thermolysis is the process of chemical transformation of the dissolved organics with or without metal compounds (salts) at moderate temperature and self (autogenous) pressure in the absence of oxygen or an oxidant. This process is a thermo-chemical precipitation and anoxic (in the absence of oxygen). Inorganic salts hasten the process of metal complexationand precipitation, just as coagulants/flocculants work at room temperature. The solid residue can be combusted or incinerated to produce energy, which can be used to generate steam. This steam can be used to fulfill the hot utility demand in the same plant.

Wet OxidationWet oxidation is a hydrothermal treatment of aqueous solutions of biologically recalcitrant and hazardous chemicals/wastes. It is the oxidation of dissolved or suspended components in water using oxygen as the oxidizer.Wet air oxidation involves the liquid phase oxidation of organics or oxidizable inorganic components at elevated temperatures (125-320 °C) and pressures (0.5-20 MPa) using a gaseous source of oxygen (usually air).Enhanced solubility of oxygen in aqueous solutions at elevated temperatures and pressures provides a strong driving force for oxidation.

Wet Oxidation

The degree of oxidation is mainly a function of temperature, oxygen partial pressure, residence time, and the oxidizability of the pollutants under consideration.

The elevated pressures are required to keep water in the liquid state. Water also acts as a moderant by providing a medium for heat transfer and removing excess heat by evaporation.

ELECTRO COAGULATION (EC)

EC is a coagulation of dissolved and suspended solids in water by using in-situ generated metal hydroxides and flocculated and buoyed-up by the gas generated at the electrodes.

It is environmentally compatible - electrons act as clean reagents, with short electrolysis times (ET).

Using electrodes (cathodes and anodes) made of dissolving metals under electro-chemical potential, in-situ coagulation and oxidation takes place.

Simple equipment which could be designed to virtually any size with an easy start-up (unlike in biological treatment) and also as a standalone system.

Very high electrochemical efficiency.

Pretreatment option to treat biologically refractory/recalcitrant wastewater, inactivates bacteria, virus and cysts and adds no harmful chemicals to water.

Very effective in removing contaminants of Potable Waters: Toxics, etc.

Often use benign (e.g., ambient) conditions of temperature and pressure.

Avoids use of chemicals reducing the problem of neutralizing excess chemicals and the generation of secondary pollution as well.

EC generated flocs tend to be much larger, contain less bound water, and thus improved filterability of the treated slurry.

A low sludge producing technique; sludge formed has quick settleablecharacteristics with easy dewaterability.

Electrolytic processes in the EC cell are controlled electrically with no moving parts thus requiring low maintenance.

Amenability to automation-the electrical variables, (I and E) are suited for facilitating data acquisition, process automation and control.

Advantages of ECT (contd.)

Black liquor being treated Flocs atop ECR

Energy recovery from pulp and paper mill effluent

Study 1: THERMOLYSISIn this work, the removal of chemical oxygen demand (COD) and color of paper mill wastewater due to the thermochemicalprecipitation of dissolved solids was studied in the temperaturerange from 20 to 95 °C using different catalysts/chemicals. The homogeneous CuSO4 catalyst was found to be the most active in comparison to the other heterogeneous catalysts under similar operating conditions. The pH value showed a pronounced effect on the precipitation process. At an optimum initial pH of 5.0, a maximum COD reduction of 63.3% was obtained with a catalyst concentration of5 kg m-3, although the maximum color removal was 92.5% using a CuSO4 concentration of 2 kg m-3. The residual copper in the supernatant works as a good catalyst for wet air oxidation of the supernatant.


The elemental analysis of the sludge, black liquor, and the supernatant shows carbon, hydrogen, and sulfur enrichment in the sludge in relation to black liquor. The heating value of the sludge is around 19.72 MJ/kg, which is comparable to that of the Indian coal (20.90 MJ/kg). Thus, a large part of the energy of the effluent could be recovered in the form of sludge with simultaneous reduction in its COD value.On the basis of the results, it can be concluded that treatment of pulp and paper mill effluent using catalytic thermolysis followed by CWO can reduce the COD of the pulp and paper mill wastewater significantly at moderate operating conditions, thus making the waste stream much less polluted. Besides this, a high heating value solid precipitate obtained during pretreatment step can be used as co-fuel in energy intensive industries.


Study 2: Electrochemical degradation

The electrochemical degradation of agri-based paper mill wastewater (black liquor (BL)) was investigated in a 2 dm3 electrolytic batch reactor using iron plate electrodes. Of the four-, six-, and eight-plate configurations, a current density of 55.56 A/m2 at neutral pH with a six-plate arrangement was found to be optimal, achieving a maximum chemical oxygen demand (COD) and color removal of 80% and 90% (175 platinum-cobalt units (PCU)), respectively. The chemical dissolution of iron was strongly influenced by pH0. Electrochemical treatment at higher pH0 (pH0 = 9) increases the dissolution of iron electrodes by an order of magnitude. An increase in salinity reduces the treatment time significantly, and the sludge settling characteristics also improve. The addition of polyacrylamide (10 mg/dm3) to the electrochemical reactor enhances the COD removal rate with a very short treatment time with excellent sludge settleability.


Specific energy consumption (SEC) reduces from 6.64 to 5.73 kWh/kg of COD removed with the addition of NaCl(625 mg/dm3). The post-treatment of electrochemically treated wastewater by chemical coagulation using alum (360 mg/dm3) along with 20 mg/dm3 polyacrylamide (PAA) further reduced COD values to <180 mg/dm3 and a near 100% (<5 PCU) color removal. An overall COD removal of 91% and color removal of near 100% could be achieved by electrochemical treatment followed by coagulation/flocculation [Mahesh et al., 2006a].


Table: Characteristics of EC Generated Sludge under Optimal Conditions and that of Indian Coal


The sludge obtained from the EC treatment of the BL at its natural pH had very good settling characteristics. PAA addition hastens the EC process, and the sludge settling rate improves considerably.

The sludge has lower ash content and fixed carbon and higher volatile matter than that of Indian coal.

Thermal analysis showed good combustion characteristics and complete oxidation of the EC process sludge at about 400 oC, with a heating value of 11.33 MJ kg-1.

The sludge can be dewatered, dried, and used in the furnace/incinerators for its heat recovery, and the ash may either be blended with organic manure for use in agriculture/horticulture or may be blended with clay/coal fly ash to make bricks/ceramic tiles for the building industry [Mahesh et al., 2006b].

Energy recovery from distillery effluent

Study 1:

COAGULATION

The removal of molasses-derived colour and COD from the biodigester effluent of a molasses-based alcohol distillery effluent treatment plant was studied using inorganic coagulants—FeCl3, AlCl3 and polyaluminium chloride (PAC). The coagulation/flocculation yield about 55, 60 and 72% COD reductions and about 83, 86 and 92% colour reductions, with the use of 60 mM/l AlCl3, 60 mM/l FeCl3 and 30 ml/l of PAC, respectively, at their optimum initial pH. The critical pH of the effluent–coagulant mixture plays a very significant role in the coagulation/flocculation process, with pH0 5.5 being the optimum for PAC.

Energy recovery from distillery effluentTable: Material balance for the metals in the sludge and the filtrate


Table: Analysis of residue obtained after coagulation


Table: Elemental analysis of the BDE, PAC sludge and the filtrate


The dried sludge was found to have medium heating value (13.8–15.0 MJ/kg) and can be used as a fuel in a combustor, furnace or incinerator. The metals, Al, Fe, Na, K, P, Fe, Mg contained in the sludge cannot be recovered economically. However, these metals will remain in the ash obtained after incineration/combustion. This micronutrient ash can be blended with the composted manure and used as the nutrient supplement for the horticultural and agricultural purposes [Chaudhariet al., 2007].High COD reduction of the waste water by flocculation with PAC may be a better alternative to the conventional aerobic treatment process of the biodigester effluent.


Study 2:

Catalytic thermolysis of distillery wastewater

The present study deals with the catalytic thermolysis (CT) of sugarcane molasses-fed distillery wastewater (DWW) using CuOcatalyst. Experiments were conducted in a 1 dm3 stainless steel pressure reactor in a batch mode in the temperature range of 100–140 ◦C and corresponding autogenous pressure with CuO mass loading in the range of 2–5 kg/m3. The initial pH (pH0) is found to have profound impact on the efficiency of thermolysis in COD removal and the optimum pH0 is found to be 2. At 140 ◦C with 3 kg/m3 catalyst loading and pH0 2, a maximum of 60% COD could be reduced.


Table: Material balance for copper


Table: Composition of DWW, filtrate and residue (T = 140 ◦C, pH 2, Cw = 3 kg/m3)


The CT process results in the formation of settleable solid residue and the slurry obtained after the thermolysis exhibited very good filtration characteristics.

The solid residue obtained at 140 ◦C and pH0 2, has a C:H atomic ratio of 1:1.08.

It has a heating value of 21.77 MJ/kg.

The residue can be used as a fuel in the combustion furnaces to recover its energy content and the ash containing Cu may be blended with organic manure and used as a nutrient-rich manure in agriculture and horticulture [Chaudhari et al., 2008].


Study 3: Catalytic thermolysis of biodigester effluent

CT of biodigester effluent was studied in the presence of CuOcatalyst in batch mode in the temperature range of 100-140 °C and the pressure range of 1-9 bar. Thermal treatment at 140 °C with a Cw of 3 kg/m3 gave a maximum chemical oxygen demand (COD) reduction of about 70% from its initial value of 34 kg/m3. The biochemical oxygen demand reduction was found to be 83% from its initial value of 6.3 kg/m3. A good amount of charred solid residue is obtained, which is enriched with carbon, giving a C/H atomic ratio of 1:0.947, versus a C/H atomic ratio of 1:1.146 for the effluent. Because of this carbon enrichment, the total residual organic carbon in the filtrate becomes substantially low, with a C/H atomic ratio of 1:2.41.


The charred residue has a lower ash content than the filtrate, which again indicates that the charred residue has much lower inorganicsthan the filtrate. Because the sulfur content of the residue is much lower than that of the filtrate or the effluent, the combustion of the residue will be environmentally safer than the incineration or combustion of either the filtrate or the effluent.The charred residue is a good fuel material with a high heating value (17.92 MJ/kg) representing 42-47% energy recovery from the digester effluent. This residue could be used as a fuel in a combustor, a furnace, or an incinerator. The ash contains more than 50% copper and is, therefore, a good copper source to enrich organic manure. This blended manure could be a very good fertilizer for agricultural crops.

Conclusion

Present study shows that catalytic thermolysis, wet-air oxidation, coagulation and elecro-coagulation in isolation and/or in combination can be effectively used to treat pulp and paper mill effluent and distillery wastewaters.

The sludge/residue produced generally has good heating value. The sludge can be dewatered, dried, and used in the furnace/incinerators for its heat recovery.

The bottom ash may either be blended with organic manure for use in agriculture/horticulture or may be blended with clay/coal fly ash to make bricks/ceramic tiles for the building industry.

Treatment processes presented entail large amount of energy recovery from the effluents, thus, making the treatment cost-effective.

Dr.I.M.mishraEnergyRecoveryfromIndustrialEffluents

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