# Thermal Hydrolysis of Sewage Sludge: A Case Study of a WWTP in Burgos, Spain
## 2 Materials and Methods
### Economic Balance and Feasibility Analysis  
The economic balance and feasibility analysis considers capital investment, net annual cash flow, payback and net present value at 25 years. Scenario 1 is taken as a reference for estimating the effect exerted when inserting different THP units. The capital investment calculation takes into account the equipment of the post-thickening section, THP, digestion section, steam production by the recovery mixed boiler, dewatering and nitrification-denitrification of sludge via nitrite. Scenario 1 does not contemplate the costs associated with the installation of the facility and the cost of equipment because this scenario corresponds to the WWTP operating under conventional mesophilic digestion. The price of electric power supply in Spain for consumers using 2000-20,000 MWh/year is considered as 92 \(\epsilon\)/MWh [47]. It is assumed that the total amount of biogas is consumed in the boiler and engines. The use of natural gas as supplementary fuel for the boiler is not considered.  
Biogas is assumed to be fully valorised in the engines once the thermal demand of THP is completely covered. The cost of transport and treatment of dewatered sludge to the drying area and composting plant was 11.51 \(\epsilon\)/t [48]. The cost of polyelectrolyte was 2300 \(\epsilon\)/t (indicated by the WWTP). Annual plant operating costs were estimated at 2.6% of the initial capital investment [49]. For the calculation of payback and net present value (NPV, \(\epsilon\)) parameters, a discount rate of 3% was adopted [50] and a depreciation period of 20 years was assumed using the exponential line method [51]. The calculation of the NPV was done using Equation (25), with constant net annual cash flows [52]. The internal rate of return (IRR) was obtained from this same equation by setting the NPV to zero. The payback period was calculated as the time necessary for the recovery of the investment outlay.  
\[\text{NPV}\ =\ -\text{I}_{0}+\sum_{\text{t=1}}^{\text{t=n}}\frac{\text{F}}{\left(1+ \text{ }\text{r}\right)^{\text{t}}} \tag{25}\]where F is the net annual cash flow (\(\xi\)), n is the total number of periods evaluated (years), r is the discount rate (%), t is the period (years), and I0 is the initial capital investment (\(\xi\)). Two different economic analyses were performed to compare scenarios (Scenarios 2-5) under the same conditions, using Scenario 1 as a reference. The first case (Case I) considers the WWTP of Burgos operating with mesophilic anaerobic digestion and evaluates the costs associated with the installation of a THP. Thus, different hydrolysis technologies were evaluated under this configuration. The second case (Case II) contemplates the different scenarios with the construction of a completely new sludge line, with this new section starting at the thickening zone (gravity thickening and air flotation thickening).  
The assumptions made for evaluating the installation of a THP into the WWTP for economic balance (Case II) were the following: Scenario 4 requires a boiler of 2000 kg/h, Scenarios 3 and 5 require a THP-B2.4 unit of 19.7 t TS/d [53] and a recovery boiler of 1200 kg/h [54]. Likewise, the volume of the first and second digester of Scenario 4 was calculated using a hydraulic retention time (HRT) of 16 days [40], resulting in volumes of 5300 and 4000 m\({}^{3}\), respectively. The THP-B6.2 plant was considered for ITHP because its treatment capacity was the closest to that required in Scenario 4.