Economic analysis of the pretreatment methods

The operating cost of the pretreatment methods is mainly correlated with the energy consumption during pretreatment. However, energy consumption cannot be the only parameter to be considered since the potential for energy recovery (Dhar et  al. 2012) as in the case of thermal pretreatment should be taken into account too. Moreover, the cost of energy required by the different methods may be different depending on the form of energy provided; for example thermal energy is required by thermal hydrolysis method and can be partially provided by the biogas transformation on-site. On the other hand, sonicators’ operation is based on electricity which is more expensive than thermal energy (Carrere et al. 2010).

In the case of sonication, the index used for estimation of the energy required is the specific energy input (SE) expressed in kJ/kg TSS and is defined as the ratio energy to solid mass according to eq. (7.4)

SE P t

= V ⋅

⋅ ∆

TSS (7.4)

where P is the power (kW), Δt is the duration of sonication (s), V is the volume of the sludge under sonication (L) and TSS is the concentration of the total suspended solids of sludge (kg/L).

The energy imparted to the sludge volume, however, is lower, due to losses from the electrical energy of the ultrasonic generator to the acoustical energy transmitted in the medium. The series of energy transformations during ultrasonic treatment is (Kobus & Kusinska, 2008): electrical → mechanical → acoustical → cavitation → thermal. The imparted energy is expressed as the acoustical energy and can be estimated from the thermal energy, assuming that the acoustical energy will finally result in heat when cavitation and bubble collapse occur. The thermal energy is the most used method for estimating

Anaerobic digestion of sewage wastewater and sludge 125 the acoustic energy in the medium and is based on the temperature change of the medium with time (Kobus & Kusinska, 2008).

On the other hand, the thermal energy imparted in the medium during thermal hydrolysis can be calculated directly based on the energy (Qs, kJ) needed to elevate the temperature of the sludge (To, °C) to the temperature of the pretreatment (T, °C) according to eq. (7.5):

Qs = ρ ⋅sl Vsl ⋅Cp⋅ (T −To) (7.5)

where ρsl is the sludge density (kg/m³), Vsl is the volume of the sludge under thermal hydrolysis and Cp is the specific thermal capacity of the sludge (4.18 kJ/kg °C). The actual thermal energy consumption is calculated based on the heat losses during heating. Moreover the recovery of thermal energy from the heated sludge should be taken into account when estimating the cost of the process.

Dhar et al. (2012) studied the effect of sonication (from 1000 to 10000 kJ/kg), thermal hydrolysis (from 50 to 90°C), and sonication at elevated temperature (combined ultrasonic and thermal hydrolysis treatment) on sludge and found that the increase of the ratio of soluble COD to total COD (Y, %) correlated to the imparted energy to the sludge (X, kJ): Y = 0.247X + 7.056, R² = 0.0801 regardless the pretreatment method used. However, the increase in the biogas production did not follow a linear correlation with the soluble to total COD ratio; Ultrasonic treatment was more effective than thermal pretreatement under the conditions tested in terms of biogas production, but the opposite was observed in terms of COD solubilisation.

Thermal pretreatment results in agglomeration and increase in particle size and this could have influenced the methane yield (Bougrier et al. 2005). The increase in temperature and the SE input did not seem to affect the methane yield either, but the combination of both methods resulted in higher yields. Similar results were obtained with volatile sulfur compounds generated. Sludge dewaterability was not improved by temperature raise and was rather decreased at the highest SE input.

The assumptions for the cost estimation of the pretreatment technologies (ultrasonic and thermal hydrolysis) were (a) the sludge temperature was 25°C, the heat recovery from the thermally pretreated sludge was 80%. The cost for dewatering, transportation and landfilling was $250/ton TSS, while for electricity and natural gas was $0.07/

kWh and $0,28/m³ respectively. The cost for biogas purification and specifically H2S removal through non regenerable KOH-AC bed was $0.0005/m³ biogas (cost per unit of biogas purification) and $12/kg H2S (cost absorbent per unit of H2S removal). The results of this economic assessment showed that (Dhar et al. 2012):

(a) ultrasound pretreatment yielded a net saving of $54/ton TSS at a moderate SE input (1000 kJ/kg TSS), while the net savings were negative in the case of the low and high SE inputs.

(b) the thermal pretreatment, at all temperatures tested (50–90°C), yielded a net saving from $45/ton TSS to $78/ton TSS.

(c) the thermal hydrolysis (at 50–90°C) combined with the untrasound pretreatment (at 1000 kJ/kg TSS yielded a net saving from $44/ton TSS to

$66/ton TSS.

Other aspects which have an economic impact in the long term, but were not included in the assessment of Dhar et  al. (2012), are the prevention of erosion of the equipment from sulfur compounds, the enhanced dewaterability of sludge after pretreatment and digestion as well as the optimization of the polymer dose and, finally, the investment cost for purchasing and installing the pretreatment equipment. Jolly and Gillard (2009), on the other hand, retrieved data from full scale applications of various technologies and estimated that pretreatment technologies as well as thermophilic anaerobic digestion enhanced dewaterability allowing the production of a sludge cake containing 25–32% Total Solids (TS). The cost of the polymer dose (decreasing as the volatile solid content decreases) as well as the cost of the anaerobic digestion liquor treatment (higher in ammonia concentration with increasing the process efficiency) were taken into account.

An important factor for the economic evaluation is the energy balance and the breakdown of the energy into thermal and electrical needed in each technology. The energy required depends on the performance of each method which vary according to the conditions (type of sludge – primary or secondary, solid content, temperature conditions and duration of treatment, etc.). Table 7.2 summarises the estimation of energy required in the case of some common pretreatment technologies and the biogas yields obtained. The main assumptions are also included where available.

In some cases, the energy estimates are obscure to decipher, because it is not clear if they concern the individual steps of the pretreatment or they refer to the whole anaerobic digestion unit. For example the electrical energy required for a mesophilic digester is 0.04 kWh kg⁻¹VS or 0.032 kWh kg⁻¹ TS according to Carrere et  al. (2010) and 0.150 kWh kg⁻¹ TS according to Jolly and Gillard (2009). These values are not comparable probably because Jolly and Gillard (2009) have considered the electrical consumption of the whole plant (consisting of the following stages: pre-digestion thickening, pre-treatment, anaerobic digestion, CPH plant, post digestion storage, post digestion dewatering, and liquor treatment).

Comparison between the different technologies with respect to the mesophilic digestion (as the control case) reveal that sonication requires much higher electrical energy than the others and all three studies agree that the energy balance is negative.

In the case of thermal hydrolysis, the assumption of Pérez-Elvira (2011) that the electrical demand is zero may be optimistic since electricity is indeed required to drive the various units of the process. In any case the electrical energy required is low compared to the thermal energy. All three studies concluded that the high thermal energy demand of thermal hydrolysis has not a negative impact on the economics since this form of energy can be recovered though the heat generated by the CPH units, the hot streams of the process itself, and if more thermal energy is needed, a part of biogas can be used in boilers (reducing the biogas available

Anaerobic digestion of sewage wastewater and sludge 127

table 7.2 Electrical and Thermal Energy required in sewage sludge treatment and biogas yields. technologyJolly and gillard (2009)pérez-Elvira (2011)carrere et al. (2010) AssumptionsElectrical/ thermal energy (kWh kg−1 tS) AssumptionsElectrical/ thermal energy (kWh m−3) Biogas yield (l kg−1vS) AssumptionsElectrical/ thermal Energy (kWh kg−1 vS)

Biogas yield (kWh kg−1 tS) None (only mesophilic digestion applied)

Input: 6% TS HRT: 18d TS reduction: 45%

0.150/NRInput: 8% TS 35°C, HRT: 17d TS reduction: 42%

NR488Input: 6% TS, 80% VS 35°C, HRT: 20d VS reduction: 40%

0.04/0.51.9 None (only thermophilic)Input: 6% TS HRT: 22d VS reduction: 56%

0.178/NRInput: 6% TS, 80% VS 55°C, HRT: 15d VS reduction: 50%

0.03/1.02.4 Thermal hydrolysisInput: 11% TS 165°C, 30 min VS reduction: 60%

0.310/ NR 11 bar steam from CHP/boiler Input: 8% TS 170°C, 30 min TS reduction: 55%

0/NR652Input: 9% TS, 80% VS 170°C, 15–30 min VS reduction: 60%

0.04/2.02.9 Biological – Enzymatic hydrolysis

Input: 7.5% TS 42°C, 15 h VS reduction: 52%

0.304/NRInput: 6% TS, 80% VS 70°C, 9–48 h VS reduction: 50%

0.03/1.02.4 SonicationInput: 6% TS VS reduction: 55.5%

0.675/NRnput: 8% TS 100 kWh m−370/NRNRInput: 6% TS, 80% VS 100W, 16s 30 kWh m−3 VS reduction: 50%

0.37/0.52.4 NR: Not Reported.

to the CHP and thus ‘consuming’ the potential electrical energy that could be produced. Pérez-Elvira (2011) showed that the process can be self sustained in the case of a 13% TS, that is, the thermal energy for the pretreatment was obtained by recovering the heat from the process itself, increasing the profitability of the combined process. The author estimated that the economic benefit of treating the sewage sludge from a population of 100,000 is €132,373/yr (8% TS inlet) and

€223,867/yr (13%TS). The biogas productivity reached 1.4 L L⁻¹ d⁻¹ compared to 0.26 observed in conventional AD systems (Pérez-Elvira et al. 2011).

The savings in energy is not the only criterion for selecting a treatment scheme.

Mills et al. (2014) studied five scenarios for conversion of sewage sludge to energy.

The core technology in all five scenarios was the anaerobic digestion process.

Thermal hydrolysis was selected as a pretreatment step in most of the scenarios.

Post treatment steps for biogas exploitation (in CHP or as biomethane) and digested sludge disposal (land application, solid fuel production or conversion to pyrolysis gas to be used in CHP) were considered as alternatives and evaluated against the conventional scenario of an anaerobic digestion unit coupled with CHP and the digested sludge be utilized in land applications. They performed a life cycle analysis (LCA) to include both inflow and outflow materials and energy of each scenario as well as the emissions to the environment on the assumption of treating 100 total dry solids per day. They also estimated the Capital Expenditure (CapEx) based on the simplified equation (eq. (7.6)) as well as the annual operating expenses (OpEx). Based on CApEx and OpEx and assumptions made for the discount rate (8%), they calculated the internal rate of return (IRR) of the investment for each scenario and considered both cases of the offer or absence of incentives by the UK state.

CapEx= k S⋅ ^{0 6.} (7.6)

where k is the cost of the asset and S is the scale assumed.

The combination of the environmental and economical impacts for each scenario indicated the thermal hydrolysis pretreatment is preferable to the conventional anaerobic digestion scenario. The IRR was estimated to be 10.6% with incentives or 4.05 without incentives in the case of the conventional case, while the application of the thermal hydrolysis pretreatment step increases these values to 12.75% and 5.98% respectively. The environmental impact was also positive in the case of thermal hydrolysis pretreatment compared to the conventional case.

The final use of biogas and the digested sludge alter both the economics and the environmental impact of the processes. In the case of biogas upgrade to biomethane (for injecting it to the grid) and use the digested sludge to land, the IRR raised up to 18.92% (with incentives), but was negative without incentives (the investor would not see a return on the investment within the operational life of the plant) and the environmental impact was the most negative of all scenarios. The reasons for this is due to the particular incentives policy of UK, as Mills et al. (2014) state, which

Anaerobic digestion of sewage wastewater and sludge 129 is rather high in this case, and poses high risks for the investment if this policy is adjusted or cancelled. The use of biomethane as vehicle fuel is a better option since the higher prices of biomethane as vehicle fuel would make the investment less independent on the incentives. The scenarios of using the digested sludge for solid fuel or in CHP after pyrolysis are comparable with high IRR (14.39% and 17.46% with incentives and 8.48% and 7.64%) and the most positive environmental impacts.

In the same line, Jolly and Gillard (2009) have concluded that the choice for the final disposal of sludge determines the economics and the payback period.

Incineration of digested sludge, despite the high capital cost, results in more positive cost balance and shorter payback period than land application. In their study, they concluded that the thermal hydrolysis and thermophilic digestion are the best choices in this respect. They admit that their conclusion on the efficiency of thermophilic digestion should be verified by other works too, since they relied their estimations on data taken from one single plant.

Co digestion of sludge with other feedstocks such as organic residues as glycerol (Athanasoulia et al. 2014), landfill leachates (Pastor et al. 2013) agricultural wastes or energy crops (Hidaka et al. 2013; Galitskaya et al. 2014) or food wastes (Serrano et al. 2014; Belhadj et al. 2014; Powell et al. 2013; Dai et al. 2013) would increase the efficiency of the process as well as the quality of the digested liquor and sludge.

The economic evaluation of developing a biogas unit within a sewage treatment plant accepting more inflows than sewage to increase the methane potential could be based on the work of Karellas et  al. (2010). They developed an investment decision tool for biogas production from biomass feedstocks. This tool requires inputs such as the feedstock characteristics, availability and their gate fees (in the case of outputs of industrial activities) or cost (in the case of biomass), the market prices for the end products (electricity, heat, deigested sludge, liquor) and additional revenues, the total capital and annual operating costs and any economic incentives (loans, existing subsidies and grants). The output of this tool is the economic evaluation of the investment in terms of essential economic indicators such as the internal rate of return (IRR) and net present value (NPV) and so on.