1. Introduction
Globally, many investments related to renewable energy are actively underway to reduce the production of greenhouse gases corresponding to the Kyoto Protocol. In the Republic of Korea (ROK), the central government is actively participating in various initiatives to increase the power generation based on renewable energy sources such as photovoltaic, solar heat, wind, small hydro, geothermal, and bioenergy. Additionally, due to London Convention, which is preventing marine pollution from dumping of wastes, waste-to-energy systems such as wastewater treatment plants (WWTP), trash incinerators and landfill gas systems have become one of major concerns for both reducing the amount of waste and saving operational costs [
1]. Compared to other systems, the WWTP can produce biogas stably with relatively constant concentration because of controllability of total solids (TS) referring to matter suspended or dissolved in wastewaters.
The typical WWTP requires digestion facilities (DF) to separate volatile solids (VS) that can be transformed into potentially biogas as a renewable biofuel from TS. As the growth of urban populations leads to an increase in TS throughput, more DF have been installed. Typically, DF can be categorized into anaerobic digestion (AD) and anaerobic co-digestion (AcoD). While AD only deals with sewage sludge (SL), AcoD can process both SL and food waste (FW), resulting in better nutrient balances and a larger yield of biogas [
2,
3,
4,
5]. As a result, existing AD facilities in the ROK have been gradually replaced with AcoD facilities, and additional DF installations have been made. The composition of these anaerobic digestion systems can be divided into single-reactor and double-reactor configurations. The single-reactor system has a structure in which the processes of hydrolysis, acidogenesis, acetogenesis, and methanogenesis are carried out in one reactor. The double-reactor system is comprised of two separate reactors, with the first reactor performing the hydrolysis/acidogenesis stage and the second reactor producing methane [
6]. The data used in this paper is based on a double-reactor anaerobic digester.
Figure 1 shows the status of biogas production facilities in the ROK. It can be noticed that the total capacity of DF increases by around 5.9% annually. Furthermore, many efforts including system upgrade from the conventional AD to AcoD have been fulfilled to enhance the ability to handle organic substrates. This leads to an increase in biogas production, as shown in
Figure 2. However, there has not been much progress in consuming biogas.
Since raw biogas consisting of mainly methane and carbon dioxide has low-concentration methane, burning out or discarding of the raw biogas may be a simple economical solution even under annual biogas production increase and the improvement of the treatment process. However, it also causes environmental issues, thus the biogas utilization system of the WWTPs needs to be enhanced for both energy saving and wastes reduction [
7,
8,
9].
In recent years, few papers have improved biogas utilization systems of the WWTPs and its monetary value estimation in [
10,
11,
12]. In [
10], authors conducted the life-cycle assessment to hydrogen generation in the WWTP and concluded that additional carbon capture and storage systems should be added to obtain economic feasibility of the WWTP. In [
11], the technological feasibility of carbon membranes for biogas upgrading was studied and its optimal operational conditions was derived. An optimum size selection of biogas-fueled micro gas turbine cogeneration systems in the WWTP was studied in [
12], which concluded that the best configuration is when the rated fuel input of micro gas turbine cogeneration systems is approximately equal to biogas production of the WWTP.
To improve the uneconomic characteristics of DF operations, previous studies have been focusing on the economic feasibility analysis and optimal size selection of single biogas utilization system authors proposed. In this paper, comparative studies regarding the economic feasibility of using various energy utilization methods are required, which are based on methods of improving DFs and methods of using the produced biogas considering energy conversion. At the same time, different characteristics and results between AD and AcoD are discussed. By comparing net present values (NPV) of several proposed utilization systems, better investment decision can be achieved. Real operational data from the public WWTP located in Sejong city of ROK is used to increase the reliability of the derived economic analysis results through reflection of a variation of the rated daily gas production and its concentration according to the weather and seasons. Additionally, to minimize the estimation errors due to uncertainties of the gas concentration and the gas selling price, a Monte Carlo simulation considering the stochastic input data is carried out. As a result, the proposed approach can lead the better decisions in selecting more suitable biogas utilization system by forecasting the ranges of possible economic values. The contribution of this paper can be summarized as follows: (1) To provide a methodology for estimating the monetary benefits of improved biogas utilization systems, (2) To propose practical options for utilizing the biogas produced in the WWTP, and (3) To calculate the NPV of these improvement options using real operating data and Monte Carlo simulation.
The rest of the paper is organized as follows.
Section 2 presents the economic value estimation model used in this study. The biogas production facility is described in
Section 3 as well as the identification of the problems. The production facilities with improved biogas utilization methods are proposed in
Section 4, and the simulation input conditions for the economic value estimation of the proposed methods are explained in
Section 5. The simulation results are presented in
Section 6, and
Section 7 draws the conclusion in this study.
2. Methodology
The NPV refers to a conversion of the time-based difference between the cost and the benefit into the present value, using the proper discount rate. An investment with an NPV of greater than zero in the entire project investment period proves that the project is viable. Furthermore, when there are several mutually exclusive cases, it may be valid to select a case with the largest NPV among those having an NPV of zero or higher [
13,
14]. The NPV output value of the economic value estimation model in this study can be calculated as
where,
Bt,
Ct,
Ci,
γ and
n are
t-year-round benefits,
t-year-round costs, initial costs, discount rates, and investment periods, respectively. The annual benefits include the renewable energy certificate (REC), system marginal price (SMP), carbon credits, environmental improvement amount, gas and hydrogen sales revenue. The annual costs include labor and maintenance costs. The initial costs include generators, electric power facilities, exhaust emission removal, leakage gas monitoring unit, membrane, gas compressor, dehumidifier, CO
2 capture system, reformer, compressed gas tank unit, installation construction cost, several business approval cost and protective relay scheme inspection cost. It is worth mentioning that this study does not consider depreciation costs for wastewater government facilities. In the ROK, social infrastructure can replace depreciation costs with repair and maintenance costs [
15,
16]. Additionally, the provision of public WWTP is relatively effortless because of the country’s small territorial size and high population density. Thus, capital expenditures are not accounted for as costs. The investment period for the project is set at 10 years because WWTP is considered to deteriorate after 10 years in the ROK. Lastly, the discount rate used in this study is defined as 5% per year.
3. Biogas Production Facilities
Figure 3 shows the view of the production facility, with (a) and (b) representing Facility A and Facility B, respectively. Both DFs are configured in a multistage digestion method and operate independently. Facility A is an AD facility, which only processes SL, while Facility B is an AcoD facility, processing both FW and SL. Since both facilities differ in microbial characteristics, economic value estimation will be conducted separately [
17].
The design of Facility A consists of two mesophilic temperature anaerobic digestion reactors, with the first reactor based on 38 °C and the second reactor operated at 35 °C. The total hydraulic retention time is 18 days. In practice, both reactors are running at an average temperature of 40 °C. On the other hand, Facility B was designed with two thermophilic temperature anaerobic digestion reactors, both intended to operate at 55 °C. The hydraulic retention time for the first reactor is 3.7 days and 14.6 days for the second reactor, with a total of 18.6 days. However, due to operational difficulties at thermophilic temperature and insufficient heating capacity, both reactors are currently operating at around 40 °C. The average methane composition of the generated gas is approximately 60% for Facility A and 64% for Facility B.
Table 1 presents the design capacity and practical usage of the FW and SL. It can be noticed that the operating rates of SL and FW in Facility B are 67% and 10% on average, respectively and the operating rate of SL is 31% in Facility A. It is worthwhile to mention that Sejong city was founded in 2007 as the new planned capital of ROK and the construction of the city is expected to be completed in 2030, at which time 500,000 people are expected to live there. Therefore, the continuous population inflow to Sejong City will be expected and the operational rates will be increasing continuously.
Figure 4 shows the biogas production and usage of Facility A and Facility B in 2021, respectively. Facility A incinerated 33,634 m
3, which is equivalent to 11.95% of the total gas production, 281,505 m
3. Facility B produced 643,364 m
3, of which 192,085 m
3 is equivalent to 29.86% of that was incinerated.
The amount of environmental improvement can be estimated through the incinerated biogas (extra gas) and can be expressed as
where
Va,
PLC,
Am and
n are volume of gas, low calorific power, hot water price from the Korea District Heating Corporation (KDHC) and boiler thermal efficiency, respectively.
Table 2 presents the calorific power of biogas, the thermal efficiency of the boiler, and the KDHC heat supply cost [
18]. Finally,
Table 3 presents the environmental improvement amount consumed in each facility as the benefits of the facility improvement, which was derived using (2) and
Table 2.
7. Conclusions
This paper presents the economic value estimation of improved biogas utilization systems in public wastewater treatment plants located in Sejong, ROK. Three operating options able to leverage the produced biogas are proposed, and then their monetary benefits, considering both actual operational data and Monte-Carlo simulation, are estimated through net present value calculations. The results indicate that e-CHP systems have the potential to be the most economically feasible models for both the AD and AcoD digestion facilities. Compared to AD, AcoD can produce a comparatively large amount of biogas, so other options, such as upgrading systems and hydrogen production systems, can also be considered. Reforming systems are an active area of research, and further evaluation may result in varying outcomes based on the country, region, and operating conditions. Additionally, since analysis results can vary depending on facilities operating rates, it is essential to check the facilities when energy recovery systems are considered.