Sustainability of the anaerobic digestion of oil refinery secondary sludge

ABSTRACT

Among the waste generated at oil refineries, secondary sludge from biological wastewater treatment processes (activated sludge systems) stands out. This paper aimed to assess the use of anaerobic digestion (AD) to treat sludge by SWOT (Strength, Weakness, Opportunity, and Threat) analysis, ranking the different factors based on sustainability criteria. Additionally, the SWOT factors were matched (TOWS matrix) to help interpret the results. AD was found to be compatible with sustainability. The results demonstrated that the strength of AD (reduced organic load) compensates for its weaknesses (need for operational control and initial implementation costs), thereby avoiding the threat (sludge composition) and making the most of the opportunity (lower disposal cost). AD and co-digestion (added with food waste) used to treat oil refinery sludge showed that around 60% of the factors analyzed were confirmed experimentally. It was concluded that AD should be considered in the sustainable treatment of oil refinery waste activated sludge, especially when mixed with other readily biodegradable wastes.

HIGHLIGHTS

• Anaerobic digestion (AD) is a potential treatment for secondary sludge from refineries.

• AD is compatible with sustainability.

• Anaerobic co-digestion of refinery sludge contributed positively to sustainable treatment.

• The strength (reduced organic load) of AD counteract its weaknesses (operational control and implementation costs).

• The threat of AD (sludge composition) should be avoided and the opportunity (low disposal cost) leveraged.

GRAPHICAL ABSTRACT

Keywords:

• Oil refinery
• solid waste
• secondary sludge
• anaerobic co-digestion
• SWOT analysis

Introduction

Petroleum is still an essential raw material and a basic component of several products. A number of stages occur from its prospection to derivative consumption. Downstream refineries are an important stage in the oil and gas industry. Fossil raw materials, largely composed of hydrocarbons, are transformed into fuels, lubricants, petrochemical products such as rubbers and plastics, and even electric energy are obtained [1]. Oil refineries are large industrial facilities, generally located near urban areas, that use a large amount of water and generate gases, wastewater and solid waste that is difficult to be treated and disposed. Wastewater treatment typically consists of a primary stage, in which sludge is settled while grease and oils rise to the surface; secondary treatment, whereby organic matter, emulsified oils and most polluting agents are removed; and an optional tertiary phase to improve effluent quality for uses other than disposal [2–5]. In the present study, only sludge from secondary treatment was considered.

The main type of secondary treatment for oil refinery wastewater is the activated sludge system, a versatile process that produces a high-quality final effluent and does not require a large area when compared to other biological processes [6]. In this system, up to 40% of the organic matter introduced can be converted into biomass (sludge), which must be continuously discharged. The cost of managing excess biological sludge (secondary sludge) is high, reaching up to 60% of the total cost of a treatment plant [7].

Oil refinery waste activated sludge (OR-WAS) has high moisture content, is an essentially organic solid waste with mineral elements, heavy metals and pathogens from activated sludge. Its management at refineries involves water removal (dewatering), followed by organic matter removal (stabilization) and disposal in industrial landfills without reuse [1]. In addition to landfills, other possible disposal processes for activated sludge include thermal treatment and anaerobic digestion (AD) [8].

Landfills require drainage systems, treatment of the leachates and gases generated, monitoring groundwater, lining or waterproofing to protect the soil, and continued monitoring after closure. Thermal treatment of sludge by incineration is a stabilization process that reduces its volume for disposal. Although the process eliminates many components of the residue, it generates polluting gases and ash that may be more toxic than the original waste [9]. Depending on the characteristics of the sludge, 10 to 30% of the dried solid waste incinerated remains as ash after treatment [10]. However, due to their complexity and high implementation and operational costs, the use of incinerators is limited to treating large sludge volumes, such as sewage sludge in densely populated areas. This technology cannot be classified as adding value to waste because its energy balance is negative due to the high moisture content of sludge.

Secondary sludge after AD is widely used in agriculture as a means of disposal for sludge from conventional sewage treatment systems [11,12]. However, oil refinery sludge from AD treatment is not indicated for agricultural use due to the presence of toxic materials. In the past, it was often disposed of in the ocean, a practice now banned in several countries, including the United States and Brazil [6,13].

AD is an alternative use for OR-WAS, as reported by Castro et al. [14]. This process involves the biological conversion of organic waste by microbial population in the absence of oxygen [15]. In addition to stabilizing the organic load of waste, AD reduces pathogens and generates products such as biogas, consisting of around 60% methane and 40% carbon dioxide [16,17], as well as organic waste rich in assimilable nutrients that, depending on their quality, can be used directly as a biofertilizer [11,12].

AD of protein-rich recalcitrant or inhibitory compounds is a slow process with low biogas production [18]. An alternative to overcome these limitations and improve performance is anaerobic co-digestion (coAD), which balances nutrients and increases the amount of substrates with greater biodegradability potential, in addition to incorporating biomass that is better suited to the biostabilization of waste [19,20]. As such, incorporating AD or coAD into the initial design of treatment plants for oil refinery wastewater could improve sludge management by adding value and minimizing final disposal.

SWOT (strength, weakness, opportunity and threat) analysis is a framework widely used for decision making about the future goals and activities of individuals, groups or companies. Waste treatment also requires analysis to determine the best strategy for the future of the plant. Several studies have used SWOT analysis for urban solid waste management [21–23], converting waste into energy [24–28] or recycling assessment [29–31], whereas Adar et al. [32] and Samolada and Zabaniotou [33] used sewage sludge, which is similar to oil refinery waste activated sludge. No studies were found that assess treatment techniques for industrial or refinery sludge. As such, this study aimed to use SWOT analysis to assess the sustainability of AD technology for treating secondary sludge generated in oil refinery-activated sludge systems, based on the literature and experimental data from a previous study [14].

Methods

The study was carried out in four stages: 1) identifying the internal and external aspects of AD, based on a literature review, to compile the SWOT matrix; 2) ranking elements of the SWOT matrix based on sustainability criteria; 3) matching information from the SWOT matrix to assess the positive and negative aspects on AD for oil refinery waste activated sludge; 4) reassessing the SWOT matrix based on primary data obtained experimentally in a previous study [14].

Identifying aspects of anaerobic digestion for SWOT analysis based on secondary data

Based on data obtained in the literature review (secondary data), a SWOT matrix was constructed to assess the application of AD for secondary sludge generated in biological wastewater treatment processes (activated sludge systems).

In the literature review, searches were conducted for data on AD of sludge in general and this information was then discussed and applied to oil refinery waste activated sludge.

The SWOT matrix was organized according to relevant factors, mainly considering the environmental or technical aspects of sludge treatment. The definitions described by Srivastava et al. [23] were used, whereby the internal factors of the process assessed are strengths or weaknesses that facilitate or hinder achieving the objectives, respectively, and external factors are opportunities or threats that may favor accomplishing the objectives or compromise the operations in question [34,35].

Ranking elements of the SWOT matrix

Sustainability criteria were identified to rank the elements of the SWOT matrix. These criteria were taken from different studies and include principles of green chemistry [36], the 12 principles of green engineering [37], green engineering principles obtained from the San Destin Conference [38], the 5 core principles of sustainability [39], and green design metrics [40] (Table 1).

Table 1. Sustainability criteria adopted to analyze the results of the SWOT matrix.

Sustainability Criterion (SC)	Identification	Description	Reference
Avoid harmful and toxic substances	SC1	Aim for less toxic and harmful reagents, intermediates, auxiliary substances (such as solvents) or by-products.	[36–38,40]
Prefer renewable materials	SC2	Minimize the depletion of natural resources, which can be achived by using renewable (materials and energy) instead of non-renewable raw materials, by integrating mass and energy flows or obtaining renewable by-products	[36–40]
Avoid waste formation	SC3	Prioritize not generating waste as opposed to generating and then treating it. Seek alternatives that minimize wastewater, solid waste and atmospheric emissions.	[36,37,40]
Promote mass efficiency	SC4	Opt for the most efficient and selective synthesis routes possible, avoiding waste. Incorporate used materials from the process to the final product.	[36,37,40]
Aim for energy efficiency	SC5	Seek energy efficiency, using as little as possible, considering that different energy sources have different impacts.	[36,37,40]
Avoid economic and by-product waste	SC6	Aim to optimize costs and resuse by-products. This also applies to the construction of treatment plants and facilities, avoiding excess size.	[37,38,40]
Adopt a safe and holistic approach to accident prevention	SC7	Safety should be addressed in and integrated approach across the entire system. Prioritize intrinsically safe designs over those involving risks that need to be addressed with safety solutions.	[37,40]
Minimize the number of steps or derivation	SC8	Prioritize simplicity whenever possible. Avoiding unnecessary steps means avoiding risks, costs and knock-on effects.	[37,40]
Consider useful life, the service of the product and dismantling installations.	SC9	The product or installation should be sufficiently durable to satisfactorily perform its function without remaining in the environment once its service life is over. This is also applied to the construction of facilities, which should be designed taking into account their decommissioning.	[36–38,40]
Aim to regionalize processes	SC10	Execution of a process must take into account the environmental, geographical, cultural, regulatory and social characteristics of each location.	[38,40]
Monitor and control to prevent pollution	SC11	Flaws and process deviations can occur even with safe designs. As such, monitoring and control must be adopted to prevent the release of pollutants or harmful substances.	[36,40]
Aim for social and environmental responsibility	SC12	Avoid social and environmental impacts, that is, harmful effects on ecosystems and communities around installations and with by-products.	[38,39]
Adopt an integrated view of the process or product	SC13	Consider not only the individual events or processes, but rather anlayze the supply chain as a whole, from material input to output.	[38]
Promote innovation aimed at sustainability	SC14	Promote innovation considering the three pillars of sustainability, that is, social, environmental and economic aspects.	[38]
Incorporate complexity into the process to achieve greater susutainability, make green choices.	SC15	This criterion can be interpreted as an exception to criteria 6 and 8. However, in this case, a larger number of steps and increased complexity are incorporated to increase sustainability.	[37,40]
Use mature technology	SC16	Processes that are technologically well-established in industry are safer because they have been used long enough to reduce initial faults, favored by experience and the safety and operational records accumulated.	[10,26,32,41]
Be economically sustainable	SC17	Costs related to acquiring materials (CAPEX), the life cycle, operations, services, maintenance and the end of useful life should be taken into account and balanced against possible returns on investment and future profits to achieve the best cost-benefit ratio.	[40]

Obs [41]. was not related to the remaining references for sustainability principles, but was included because of its relevance as a criterion and citations in environmental management studies.

The elements identified in the SWOT matrix were rated from 0 to 10 for each of the positive and negative factors (strengths, weaknesses, opportunities and threats) to obtain a single importance metric. Scoring was proportional, based on the established sustainability criteria, as shown in Table 2. Higher scores (more important) were attributed to factors that met the sustainability criteria and low scores (less important) for those that did not. The ad hoc method proposed by Araújo et al. [40] was used to analyze the importance metric, encompassing the severity, impact and/or tendency of each factor.

Table 2. Scores used based on assessment criteria to rank elements of the SWOT matrix.

Score	Importance
10.0	Critically important
7.5	Very important
5.0	Important
2.5	Somewhat important
0	Unimportant

In SWOT analysis, four elements are assessed: strengths (internal factor): the strong points that facilitate achieving project objectives (in the AD case), that is, qualities and human, administrative, technological and economic capacities; weaknesses (internal factor): the weak process points that constitute internal obstacles to achieving objectives; opportunities (external factor): situations in the environment external to the process that could favor achieving objectives and lead to growth; threats (external factor): situations in the environment external to the process that could negatively affect the successful achievement of objectives, that is, endanger the activity.

Next, the strengths, weaknesses, opportunities and threats were ranked by adding the importance values for each individual factor of the SWOT matrix [42, 43]. The readiness of AD technology was determined by adding the final importance scores of each category according to each quadrant of the SWOT matrix (grid). The final scores of weaknesses and threats were added for quadrant I (survival stage), strengths and threats for quadrant II (maintenance), weaknesses and opportunities for quadrant III (growth), and strengths and opportunities for quadrant IV (development) [23,34,35]. The highest value obtained in the four quadrants determined the readiness of the technologies assessed in an integral approach.

Matching elements of the SWOT matrix

All the elements of the SWOT analysis were matched in a TOWS matrix to assess the competitiveness of the proposed technology for AD. The TOWS matrix consists of four strategic combinations: SO, WO, ST and WT strategies. Four quadrants in a 2 × 2 matrix were combined to leverage the strong points and overcome the weaknesses, as followed: (Quadrant I) strengths and opportunities; (Quadrant II) threats and strengths; (Quadrant III) opportunities and weaknesses; and (Quadrant IV) strengths and weaknesses. Matching was performed with a view to using one factor to minimize or improve another, such as counteracting negative with positive points or leveraging improvements through a combination of positive factors. This analysis was conducted as follows: using strengths to leverage opportunities (SO) (Quadrant I), using strengths to counteract threats (ST) (Quadrant II), eliminating weaknesses by leveraging opportunities (WO) (Quadrant III), and strengthening weaknesses to reduce threats (WT) (Quadrant IV) [42].

Reassessing anaerobic digestion for oil refinery sludge based on primary data

The SWOT matrix used to assess AD was compiled based on characteristics reported in the literature (secondary data) for sludge treatment in general. In other words, in addition to using secondary data, the limitations or peculiarities of the studied waste, namely oil refinery waste activated sludge, were not considered.

The use of experiments is also relevant to validate secondary data. The secondary data sources were compared with the primary data obtained from a previous study [14]. Thus, an analysis was conducted to determine whether the information obtained in the SWOT analysis was confirmed, refuted or inconclusive for OR-WAS and if points that contrasted with the literature were included in the SWOT matrix for general sludge. The SWOT matrix was validated through factor by factor comparison with the results and technical observations of the experimental stages.

Results

This study aimed to assess anaerobic digestion (AD) technology for treating secondary sludge generated in oil refinery-activated sludge systems. To that end, based on a literature review, the internal and external aspects of AD were identified; SWOT matrix aspects were compiled; SWOT matrix elements were ranked based on sustainability criteria; SWOT matrix information was matched to assess aspects of AD for oil refinery waste activated sludge; and the SWOT matrix was reassessed based on primary data obtained experimentally [14].

SWOT matrix for anaerobic digestion: assessment and ranking based on secondary data

Figure 1 presents a list of the internal and external factors indicated for each element of the SWOT matrix. In an initial analysis, AD exhibited more items related to strengths and opportunities (advantages) than negative aspects (disadvantages).

Figure 1. SWOT matrix for anaerobic digestion (AD) of sludge in general.

Discussion

The sustainability of solid waste treatment and disposal depends on multidimensional factors (economic, environmental, technical and social). A technology becomes sustainable when it is technically adequate, financially viable, economically beneficial and socially acceptable [21]. As such, values were attributed to establish the most sustainable factors and those that contributed to the non-sustainability of AD. These values were determined according to sustainability criteria (Table 1) and technical aspects defined in the literature.

Figure 2 shows the importance ranking results for each SWOT analysis element. The rankings attributed to the different factors based on sustainability criteria (SC) are discussed below.

Figure 2. Importance ranking of the strengths (A), weaknesses (B), opportunities (C) and threats (D) of anaerobic digestion (AD) identified by SWOT analysis. S1= Biogas production, S2= Mature technology, S3= Odor reduction, S4= Supplies the operational energy demand, S5= Reduces organic load, S6= Occupies a small area, S7= No drying or dehydration, and S8= Pathogen reduction. W1= Need for post-treatment and purification, W2= Operational control, W3= Initial energy expenditure, W4 = Initial cost, and W5= Retention time. O1= Biogas commercialization, O2= Digestate used as soil conditioner, O3= Reduces greenhouse gas emissions, O4= Final disposal costs, O5= Waste combinations, O6= International legislation, and O7= Location restrictions. T1= Type of material, T2= Climate conditions, T3= Specific legislation, and T4= Leaks and explosions.

Strengths

The main objective of AD is to control pollution by reducing the organic matter concentration. This factor (S5) was considered a critically important strength (Figure 2A). According to the literature, organic matter can decline by up to 65% [10,32,44]. The application of AD in sludge treatment is well-established (SC16). Any organic matter must be treated and its disposal is prohibited (SC12), making treatment processes such as AD more important (SC3).

AD requires a smaller area than landfill disposal, a strength (S6) deemed very important since the process provides an alternative to the unnecessary use of areas that could be employed for other purposes. Thus, in terms of sustainability, economies of scale are stimulated because less space is wasted (SC6). Landfills are generally located far from urban centers and transporting waste to these sites negatively affects the environment and the economy (SC12 and SC13) Moreover, AD units are easy to decommission (SC9), whereas landfill sites can become unusable and may devalue the surrounding area for years [10].

The production of biogas (S1) and its application to supply the operational energy demand (S4) are also considered very important. Factor S1 adds the most value to AD because it allows waste to be converted into clean energy (SC2, SC6, SC13 and SC15) [16,45], thereby promoting opportunity O1 (Biogas commercialization).

Promoting a closed-loop energy system by using biogas to supply the energy requirements of the process is an interesting proposal, since one of the greatest energy needs is heating during the operation. The fact that no external energy sources are needed (SC6) and the substitution of nonrenewable energy are also significant results. Using biogas outside the AD unit would be more costly and complex due to the need to transport the gas. On the other hand, it would difficult to fully meet the energy needs of the unit [45].

Anaerobic treatment of waste is typically followed by dewatering and/or dehydration of the digested sludge. However, Adar et al. [32] found that, in certain situations, digested sludge can be applied directly without the need for these processes. This is an important point because it helps reduce the number of steps in the process (SC8) and as such, lowers the costs (SC17) and energy expenditure related to drying (SC5 and SC6).

An important strength is that AD is a mature technology (S2). According to Velho et al. [41], processes that are technologically well-established in industry are considered safer because they have been used long enough to reduce or eradicate initial faults and inherent problems (SC16). The extensive knowledge of AD, whose benefits have been reported since the 1960s [46], helps counteract or minimize the impact of different weaknesses, threats and waste (SC6, SC7 and SC11). The process, its limitations and influencing factors have been widely studied [44].

Another important factor in AD is that it reduces the odor of the digested sludge (S3) in relation to its untreated ‘fresh’ state [2,10]. Although this is not decisive in the success or impracticality of AD, it is considered a strength because it can influence the location of treatment plants, particularly in terms of public opinion regarding opening new facilities in nearby urban areas. It is important to note that landfills typically emit odors resulting from waste decomposition and leachate formation [47,48]. When used as a form of disposal for sludge, combustion also produces odors resulting from gas emissions during burning, even when performed on a small scale [49].

The final strength listed is pathogen reduction, deemed only somewhat important (S8). Although AD does not eliminate the need for disinfection, which is necessary for the type of waste treated, it provides a degree of pathogen removal [3,5,7,11]. This strength is considered an additional advantage because it lowers costs by reducing pathogenicity and harm to human health in the event that the final waste remains untreated (SC11 and SC12).

Weaknesses

The weaknesses ‘high operational control’ (W2) and ‘high initial implementation cost’ (W4) were deemed very important (Figure 2B). The former indicates that the process should be periodically monitored and performed by trained personnel to obtain good results, with any change in operating parameters potentially compromising treatment and reducing efficiency. The latter (W4) is related to cost, which is considered high when compared to landfills, but lower than that of combustion [8,50]. Both these elements occur primarily during implementation of the process, where construction of the treatment plant involves high costs and energy expenditure, and are related to sustainability because they compromise SC6 and SC17, which address economic aspects.

The weakness related to energy expenditure at the beginning of the process (W3) [7,32] refers to the need to raise the temperature of the reactor contents to optimal values. This initial energy expenditure counteracts the benefits of AD treatment and is characterized as wasted energy (SC5). However, it was only considered somewhat important because energy use tends to decline during the process and the resulting biogas can be used to supply energy demands, as indicated in S4. This factor contributed significantly to the previously mentioned initial costs of AD (W4).

In order to use the biogas produced, additional steps must be included (SC8) to purify it, which is considered a weakness (W1) of AD. Similarly to energy expenditure at the beginning of the process, this factor can be minimized and was therefore deemed somewhat important. According to SC15, including biogas use in the process is a green choice [37,40] because it improves sustainability, in line with SC4, SC6, and SC13, and has become an important strength, as previously mentioned.

A disadvantage of AD is the average retention time of 30 days [3,5]. This is a weakness, since long retention times mean that larger volumes of waste may require more treatment plants, which need additional space. Nevertheless, it is not considered important because it does not directly contradict SC or prevent AD.

Opportunities

Reducing the mass and volume of waste for final disposal is considered a critically important factor (O4) [2,3,5,7,10] (Figure 2C) and benefits the process by lowering transport costs (SC17) because there is less waste than usual (SC3), which complies with circular economy recommendations for minimizing waste [51]. This is also in line with SC12, since transport is generally by road and contributes to greenhouse gas (GHG) emissions [52], with reduced transportation benefitting the environment.

The opportunities listed as very important include the possibility of commercializing biogas (O1), which can be converted into electrical energy and provide economic, environmental and social benefits [8,26,32]. This type of energy can be even more significant in the disadvantaged communities of poor and developing countries.

The use of AD technology and biogas reduces GHG emissions (O3), characterizing an important strategic opportunity (SC17). The use of biogas, which contains methane, complies with several sustainability criteria (SC15) because it prevents the gas from being released into the atmosphere (SC6). Methane is an important GHG and a greater pollutant than carbon dioxide [53].

Another opportunity classified as very important is the use of digested sludge as a soil conditioner (O2). Once the digestate has been stabilized, it can applied to non-agricultural soil in the same way as sewage sludge is used for agricultural soil [11,12]. Both these factors (biogas commercialization and stabilized sludge application in soil) are prioritized as by-products in waste reuse and in line with SC2, SC4, SC6, SC9, SC12, SC13, and SC17.

Factors O5 and O6 are related to the fact that AD allows for combined wastes, characterizing co-AD. The inclusion of different wastes for combined treatment with sludge involves the same benefits and drawbacks of conventional digestion. When their chemical composition and biodegradability are compatible, mixing two or more wastes can improve the characteristics of the final product, even if one of the materials exhibits low biodegradability. This can also reduce the number of treatment stages, since waste that would normally follow different disposal routes can be treated together (SC8), providing economies of scale, lowering costs and saving on personnel (SC6), in addition to improving treatment efficiency in some cases (SC4). Anaerobic co-digestion is a form of optimized AD increasingly used worldwide for the simultaneous treatment of different waste mixtures [19,20,44,54].

Both these opportunities (O5 and O6) were considered somewhat important and indicate a growth trend. European countries and the United States use AD for waste treatment, with specific legal and government incentives as an internationally recognized sustainable process (SC2, SC12, and SC14) [51,55]. Historically, emerging and developing countries copy the technological and environmental advances of their developed counterparts. As such, these countries are expected to expand the use of AD in solid waste treatment.

When well-operated, anaerobic digesters can be installed in any geographic location (O7) because they have little impact on the surrounding area and do not affect the soil or occupy a large area. Most operationally stable AD facilities are not harmful to the local population, in accordance with SC12 and 16. Although it cannot be considered a negative characteristic, climate is a limiting factor and will be discussed below in the section on threats [8].

Threats

Figure 2 2D shows the results of importance rankings for the threats identified in SWOT analysis. Recommendations on the risk of leaks and explosions (T4), although unlikely to occur if safety guidelines are followed [10], are contained in SC7 and were rated critically important. Despite the rarity of these events, the threat to human life means they fall under social responsibility, accident prevention and the use of harmful substances, which includes the products generated (SC12, SC7, and SC1, respectively). Additionally, they contribute to increasing the greenhouse effect, constituting an environmental threat [53].

Significant dependence on the type of raw material (T1) technically and operationally restricts the process studied and was therefore classified as very important. In general, even when the digester is fed the same type of waste, its composition varies. In some cases, its composition makes digestion unfeasible (SC17) or leads to the formation of compounds that compromise digestion (SC6), precluding treatment [3,5].

Another threat is the restrictions imposed on the process by climate conditions (T2), considered only somewhat important. AD may be not be economically viable in locations with a low ambient temperature (<20°C) [21,32] because a large amount of energy is required for heating. This factor can be minimized by installing anaerobic digesters in regions with a temperate climate (SC13 and SC10). Given that AD is a mature technology (SC16), it is well-known that high temperatures promote greater efficiency and less energy consumption for heating (SC6), making the process more sustainable [3,5].

The third threat (T3) was deemed somewhat important because the scenario is unclear and related to national policy. To date, the are no specific regulations in Brazil that govern industrial waste treatment by AD (established conditions, incentives and/or restrictions for its use). However, the technology is promising in the treatment of waste and its reuse in energy generation, favoring sustainability and complying with SC2, SC6, SC12, SC14, SC15 and SC17 [8].

Anaerobic digestion technology stage

Following analysis of the factors presented in Figure 2, the importance scores attributed to each factor were added and the values obtained are shown in Table 3. The strength values (Figure 2A) were added and the same calculation was made for weaknesses (Figure 2B), opportunities (Figure 2C) and threats (Figure 2D). Analysis of the SWOT matrix shows higher scores for the positive points (strengths and opportunities) of AD used to treat oil refinery waste activated sludge, indicating that the technology is beneficial.

Table 3. Sum of the scores of SWOT factors and their matrix percentages for anaerobic digestion of oil refinery waste activated sludge.

Factors	Score	Matrix %
Strengths	52.5	34.4
Weaknesses	27.5	18.0
Opportunities	47.5	31.1
Threats	25.0	16.4

Matching the SWOT factors

The factors identified in SWOT analysis were matched in a TOWS matrix, an important tool to identify the readiness of AD technology and devise action plans to make the most of its strengths and prepare for possible problems [34,35].

To that end, the ranking of the factors obtained by adding the scores attributed in the previous stage (Table 4) was considered for each matched pair: S-O, S-T, W-O, W-T, forming quadrants I, II, III and IV, respectively (Figure 3).

Figure 3. Result of matching factors of the SWOT matrix of the anaerobic digestion of secondary sludge.

Table 4. Number of confirmed, refuted and inconclusive elements for SWOT analysis of oil refinery waste activated sludge anaerobic digestion.

Element	Confirmed	Refuted	Inconclusive	% validated for OR-WAS
Strengths	4	3	1	50.0
Weaknesses	3	1	1	60.0
Opportunities	4	1	2	57.1
Threats	3	1	-	75.0
Total	14	6	4	58.3

In order to understand the importance of identifying the readiness of the technology studied, the relationship between internal and external factors is analyzed to provide an indication of the readiness levels of the process in question [23,34,35].

Application of this methodology demonstrated that AD for waste treatment exhibits mostly strengths and opportunities, falling into quadrant I (development), with 100 points (Figure 3). The main characteristic is leveraging opportunities through strengths; in other words, strengths can help make the most out of market opportunities, corresponding to a sustainable treatment technique developed and indicated for waste. When comparing techniques currently used for waste treatment and disposal, such as landfills and combustion, neither of which comply with the principles of sustainability, AD emerges as an important alternative for waste that can be digested [8,51].

The effectiveness of matching and weighted sums is based on using one factor to minimize another or leveraging improvements through a combination of positive factors, can be applied in all the quadrants. Some points were matched and discussed by quadrant, to better understand the system in terms of strategic planning for environmental management.

For example, in Quadrant I – strengths x opportunities, O6 (legal incentives) can leverage S6 (occupies a small area). Increased pressure from the population against using large areas for waste disposal may promote a rise in the number of laws to address the issue. The use of large areas for waste treatment, such as landfills, requires topographic changes and continued monitoring of surface water and the surrounding area for years [56]. Thus, pressure from the external environment (opportunity) contributes to leveraging the strength of not requiring large areas and all the resulting impacts.

In Quadrant II – strengths x threats, S2 (mature technology) minimizes T4 (risk of gas leaks and explosions). Given that AD produces biogas composed primarily of methane (CH₄), gas leaks may occur, resulting in explosions in the presence of an ignition source. However, because the technology is well-known and viewed by engineers as the main treatment route for the stabilization of sewage sludge, these risks have been widely studied and expertise developed to prevent them, such as using exhaust fans and other safety measures [10].

In Quadrant III – opportunities x weaknesses, O4 (reducing mass and volume to lower subsequent costs) counteracts W1 (need for post-treatment). A weakness of the process is that some form of post-treatment is often needed before final disposal in order to comply with legislation, even when the waste has been stabilized or polluting agents partially removed. An opportunity that can be used to counteract this weakness is that the final mass and volume are lower than initial values, reducing post-treatment costs [1].

In Quadrant IV – weakness x threats, W3 (initial heating) can minimize T2 (limited by climate conditions in the area). The energy input in the startup phase of AD is far greater than that required during operation. This is due to the rise in temperature to trigger important chemical reactions for AD [7,32], deemed a weakness. Another important point is the location of the digester; regions with naturally high temperatures should be preferred to reduce energy expenditure for heating during the process, for example in tropical countries.

Reassessing the SWOT matrix based on primary data on anaerobic digestion

Primary data, obtained in a previous study [14], was used to reassess AD application in the treatment of oil refinery waste activated sludge. In experiments to obtain primary data, characteristic points of the technique were confirmed and some elements in the SWOT matrix were not. These require further research or methodological adjustments to achieve the expected results. Table 4 shows the confirmed, refuted and inconclusive elements for SWOT analysis of OR-WAS anaerobic digestion.

Strengths

In the present study, the element with the highest number of factors was strengths (S), that is, AD exhibited more positive and internal points. The main objective of AD is to reduce polluting agents and organic matter concentration (S5) [2,3,5,44]. In experimental studies [14], a 51.8% decline in volatile solids (VS) was obtained in coAD of sludge added with food waste (FW), confirming this strength. Mono-digestion of OR-WAS without food waste resulted in less VS removal (31%).

The values obtained are lower than those reported in the literature (around 60%) for sewage sludge [2]. This can be attributed to the different composition of the two sludges and the fact that the OR-WAS used here was obtained from an extended-aeration activated sludge system. In this system, the sludge is partially stabilized, which hampers additional VS reduction because the easily biodegradable sludge components are oxidated during wastewater treatment.

Biogas production (S1) is the strength that adds the greatest sustainability value to the process [3,5,16], since waste that would typically be disposed of is converted into energy. In all the experiments conducted by Castro et al. [14], biogas was produced in mono-digestion of OR-WAS and co-digestion of OR-WAS and FW, obtaining a higher biogas yield (80.7 mL biogas/g VS_added) in the latter with OR-WAS (1:4 dilution) and FW at a proportion of 80:20% v/v. In other words, another strength reported in the literature [19,54] was confirmed. When converted into energy, biogas can supply the energy needed for heating [16,32], as mentioned for S4, providing greater savings and environmental gains. S4 was not entirely confirmed or refuted in bench-scale experiments because energy conversion and its effectiveness were not measured. However, this would depend on significant biogas production containing high percentages of methane [16], which could not be assessed here.

AD of sludge is typically followed by dewatering and/or dehydration (S7) to reduce mass/volume, which, according to Adar et al. [32], would not be necessary depending on the application or final disposal of the digested sludge. For example, final disposal of digested sewage sludge in agricultural soil does not require dewatering [2,10]. This requirement varies according to the characteristics of the final waste and its disposal, and dispensing with this step would be beneficial in terms of minimizing costs and the number of treatment stages. However, since OR-WAS is generally disposed of in industrial landfills, dewatering/dehydration cannot be eradicated, thereby refuring this strength.

According to the literature [2,10], the odor of the digested sludge may also improve (S3). This could not be measured here because the odor of secondary sludge from an extended aeration activated sludge system is not significant since it is partially stabilized. Nevertheless, a change in odor was perceived for the final residue of all the AD experiments conducted [14]. Odor is more marked in large-scale operations with greater quantities of waste. As previously mentioned, odor reduction may influence the location of treatment plants and social aspects related to the surrounding population. As such, this factor was defined as inconclusive in bench-scale experiments.

Another strength of AD is pathogen reduction (S8). In the case studied, although OR-WAS contains a portion of sewage sludge, it is highly diluted in the large volume of industrial waste. Additionally, the characteristics of industrial wastewater do not facilitate pathogen activity, meaning this factor was considered to be of little importance to AD of OR-WAS. Given that the number of pathogens was deemed insignificant or nonexistent, no testing was performed in the present study, making it impossible to determine whether pathogen reduction occurred [14].

Weaknesses

The greatest weakness of the process is that the main by-product, biogas, can only be used after purification (W1) [10,16,26,32], making it economically unfeasible, albeit favorable from and environmental perspective. Biogas has a heterogeneous composition and contains up to 60% CH₄ [7,10]. Methane must be separated and concentrated due to its calorific value, which is effectively used in the process or commercialized when converted into electrical energy. Since the experimental study could not quantify the methane % in biogas, purification difficulties cannot be confirmed. Further research is needed to determine the extent to which this weakness affects the benefits of AD of OR-WAS.

As reported by Castro et al. [14], high operational control (W2) is needed to ensure satisfactory treatment and considered a weakness because it can compromise the entire process.

One of the main challenges in promoting more sustainable technology is reconciling social and environmental benefits with economic viability. Although AD is generally considered a low-cost process, particularly in relation to combustion [8,50], the initial high cost should be taken into account. On a real scale, the temperature of the entire volume of the reactors must be raised for heating, which is deemed a weakness from both energy (W3) and economic perspectives (W4). On a bench scale, this point was not significant.

Depending on the treatment strategy used and the volume treated in a real-scale continuous system, prolonged retention times can be considered a weakness (W5). In bench-scale experiments [14], a 30-day retention time was used, as recommended in standard biochemical methane potential (BMP) methodology. This time can be shortened, under optimal conditions and at greater cost, or extended, depending on the type of waste treatment and the use of less stringent and more economical parameters.

Opportunities

One of the main opportunities is reducing the mass and volume of waste, which lowers transport costs and minimizes the amount of waste sent for final disposal (O4). Castro et al. [14] reported VS reductions of up to 31% in mono-digestion of OR-WAS and 51.8% in co-digestion of OR-WAS with FW, confirming lower mass after AD.

Analysis of O1 and O3, related to biogas commercialization and the incorporation of carbon credits, could not confirm success. Despite obtaining a yield of 80.7 mL biogas/g VS_added, the authors were unable to determine the composition of the biogas, precluding confirmation and comparison.

Commercialization is only possible for biogas with a high methane content, whose use depends on its calorific value. According to Andreoli et al. [10], biogas containing 70% CH₄ has a calorific value of approximately 23,380 kJ/m³ or 6.5 kWh/m³, corresponding to 63% of the calorific value of widely used natural gas. In addition to energy generation, carbon emissions can be lowered through incentives for projects based on the clean development mechanism (CDM) [26,32].

According to the results, a highly beneficial opportunity that was confirmed is the ability to combine wastes (O5) through co-digestion. The addition of food waste increased volatile solid removal from 12.9% (in mono-digestion of OR-WAS in a 1:4 dilution) to 51.8% (in co-digestion of OR-WAS 1:4 with 20% FW) [14]. Thus, co-digestion was beneficial and represented a leveraged opportunity.

Another noteworthy point is the reduced mass of organic matter in all the experiments, which could be used as fertilizer (O2) [11,12,32]. However, the composition of industrial sludge means it is not typically used as a soil conditioner. As observed in characterization, OR-WAS contains low concentrations of cadmium and lead [14], precluding its application in agriculture. As such, this opportunity was not valid for OR-WAS in the present study.

International legislation on AD is more advanced (O6). In Europe, for example, the number of AD plants has grown, largely because some countries have banned landfilling of organic waste [3,5,55]. Thus, while developing countries and other nations are expected to follow this disposal route in the coming years, it is directly dependent on difficult-to-measure political interests and efforts.

The social aspects of the technology should also be taken into account. In accordance with O7, there are no location restrictions for AD. None of the experiments identified points that counteracted the benefits of other more widely used technologies. AD does not involve environmental pollution, as occurs when waste is burned, nor it is subject to the public opposition observed for landfills in major urban centers, either due to odor or the large area occupied [44].

Threats

The main threat, confirmed in tests, is that good results depend on the characteristics of the feedstock (T1). Castro et al. [14] obtained the best results under specific experimental conditions, proving that waste type and concentration directly affect the outcome of AD and that conditions are highly sensitive and decisive. In other words, although the technique is suitable for OR-WAS and food waste, it cannot be used for certain types of waste.

The risk of gas leaks and explosion (T4) is one of the most serious threats because it endangers human lives. This factor is worthy of attention and the chances of it occurring must be mitigated. It is impossible to compare the study in bench-scale tests with full-scale reactors containing hundreds or thousands of m³ of waste, since both scale and the amount of methane influence the process. Explosions, generally after gas leaks, only occur when biogas with a high methane content is exposed to a heat source [10,16].

Temperature, which was strictly controlled in all the tests, is critical to ensuring complete AD reactions [5,15] and directly related to T2, despite assuming that the digester can be geographically located in any environment without significantly impacting large urban centers [44].

However, in terms of the climate, this factor can be deemed a threat. Under a cold climate, digesters require a high initial energy input for heating and to maintain the temperature throughout the operation. This raises the costs and makes the process unfeasible because the energy expended will be greater that that obtained from the biogas produced and outweigh the benefits of treatment itself [5,15,32].

Conclusions

The survey of solid waste treatment to oil refinery waste activated sludge indicated that anaerobic digestion (AD) is not used, despite being a mature technology. SWOT analysis of AD confirmed the process as a favorable technology.The most prominent strength was the lower volatile solids removal than that typically observed in AD of sludge alone, but with satisfactory values in anaerobic co-digestion of sludge added with food waste. AD is heavily dependent on the flow rate of feedstock, an important threat. With respect to opportunities, the technology is feasible on an industrial scale for oil refinery waste activated sludge, particularly for biogas use. As such, AD should be considered in the treatment of oil refinery waste activated sludge, especially when mixed with food waste.

Author contribution

This paper is part of the master’s thesis of Tayane Miranda Silva de Castro that was supervised by Magali Christe Cammarota and Elen Beatriz Acordi Vasques Pacheco. All authors have read and approved the manuscript.

Acknowledgments

The authors thank the Coordination for the Improvement of Higher Education Personnel (CAPES), the National Scientific Research Council (CNPq) and the Research Support Foundation of Rio de Janeiro state (FAPERJ).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The data that support the findings of this study are available from [email protected]

Additional information

Funding

The author(s) reported there is no funding associated with the work featured in this article.