JSDEWES: Anaerobic Codigestion of Sludge and Food Waste for Enhanced Resource Recovery

WAS in its original form emits a bad odour and consists of biomass and bacterial culture produced during the ASP stage. Furthermore, WAS contains harmful heavy metals (Cd, Cr, Hg, Pb), refractory organic compounds (chlorophenols, pesticides, nitro-aromatic compounds), and nutrients such as N and P [15]. Moreover, hazardous pollutants, including endocrine-disrupting compounds, nonylphenol, and bis (2-ethylhexyl) phthalate, are retained in the sludge and may cause severe environmental risks [16]. The biomass consists of 30% proteins, 40% carbohydrates, and 30% lipids in particulate forms [17]. The general physicochemical properties of WAS are presented in Table 1.

WAS consists of large amounts of water and extracellular polymeric substances (EPS) as presented in Table 2 [11]. The EPS constitutes a major part of the WAS organic fraction and is likely sourced from the ASP microbial activity or the influent wastewater. Polysaccharides (PS), proteins, and humic substances account for up to 80% of the EPS composition, in addition to lipids, nucleic and uronic acids, and some inorganic complexes [17]. EPS occurs in the interior part of microbial aggregates and outside of cells and is divided into three categories consisting of tightly bound (TB-), loosely bound (LB-), and slime (S-) EPS [18]. A representative diagram of the EPS structure is depicted in Figure 1.

The LB-EPS (loosely bound polymers, sheaths, and condensed gels), which extends from TB-EPS, have a porous dispersible structure, while the TB-EPS (attached organic materials) comparatively adheres to the bacterial cell surface inside the sludge flocs [19]. The S-EPS, which consists of slimes, colloids, and soluble macromolecules, is uniformly distributed in the aqueous phase. The surface physical and chemical properties of WAS matrices are governed by the gel-like, three-dimensional EPS biopolymers, which provide protective shielding. The shielding prevents cell lysis and rupture, thereby influencing the functional integrity, flocculation, strength, biodegradability, and dewaterability of the sludge [20].

The redress of the excess WAS to reduce environmental pollution poses a major challenge to wastewater treatment plants [11]. Costs for treating and disposing of WAS account for up to 60% of the total operation cost, and in terms of greenhouse gas emissions (GHG), sludge management accounts for >40% of GHG emissions from wastewater treatment plants (WWTPs) in South Africa [21]. A survey conducted by Snyman [22] on 72 large WWTPs in South Africa revealed that final disposal methods of WAS are frequently on-site, with the majority of sludge used/disposed of from AD. Sludge on-site disposal means the direct application on land and stockpiling. Local municipalities, farmers, and other businesses might benefit from the filtrate and/or dewatered sludge (if the technology is available). Dewatering is done by centrifuge, drying beds, or mechanical belt filter presses. The sludge can be used in several ways, including converted to compost for crops, applied at the bottom layer for golf courses, or used to grow instant lawns. The sludge can alternatively be exchanged for bulking agents with local contractors [22]. A sustainable treatment method for the WAS is thus needed.

FW contains both high moisture and organic content. The moisture content ranges from 80 to 91%, while the pH is typically acidic and ranges from 4 to 6.5, as characterised in Table 5 and Table 6. The decrease in pH value is greatly affected by the amount of time FW is stored [26]. The general characteristics of FW, outlined in Table 6, show that FW is acidic upon receipt and has a high moisture content of 82.5%. The VS/TS ratio above 90% and C/N values of approximately 18.3 confirm its biodegradability, making it highly suitable for anaerobic digestion [27]. FW generally constitutes 0.5 of carbon, 0.07 of hydrogen, 0.3 of oxygen, 0.028 of nitrogen, and 0.007 of sulphur.

In South Africa, landfilling is considered low-cost and the most practical food waste management method [33]. However, factoring in the lack of land near areas of waste generation makes landfilling expensive, and with the forecast of the closure of many landfill sites in South Africa, landfilling is not a sustainable solution [33]. Furthermore, organic waste in landfills undergoes anaerobic digestion and consequently releases GHG emissions, high in concentration of methane and carbon dioxide [34]. The decomposition of FW results in leachate that can potentially seep into water bodies and pollute them [35]. The increased costs of landfilling have also made landfilling a financially wasteful exercise [36]. Landfilling is banned in Canada, Germany, Sweden and many other countries; the banning of these practices is becoming a priority in South Africa as well [24] [37].

The average GHG emissions in the food supply chain are 2.8 to 4.14 tonnes of CO2e per tonne of food, according to Oelofse and Nahman [24]. Agricultural production, manufacturing and processing stages contributed between 1.95 and 2.29 tCO₂e per tonne of food, distribution and retail: between 0.1 and 0.8 tCO₂e per tonne of food, consumption: between 0.3 and 0.6 tCO₂e per tonne of food and end-of-life (landfill): For every tonne of food, 0.45 tCO₂e is produced indicating that the inefficiencies in the supply chain could contribute 4.14 tCO₂e per tonne of wasted food which would have a considerable impact on South Africa's greenhouse gas emissions footprint [24]. According to the DEA [38] report, 9.3 % and 4.3 % of South Africa's greenhouse gas emissions can be attributed to agriculture and the disposal of organic waste, including FW.

Municipalities must adopt technologies and processes that convert organic waste to biogas and fertiliser. Given the Circular Economy and Sustainable Processes, it is important to use waste produced as a resource. For several technologies, anaerobic digestion is considered for the recovery of energy and may be used to convert organic waste into biogas or fertilisers that can be used in agriculture [39].

Whereas anaerobic digestion has become an indispensable process in modern wastewater treatment plants, limitations are inherent due to the WAS properties [46]. The limitations include low hydrolysis efficiency and long hydraulic retention time (HRT) [47]. The low hydrolysis efficiency is attributed to the complex structure of EPS in the WAS. In particular, the walls of microbial cells are sufficiently thick to impede effective biodegradation in internal organics via normal AD processes [48]. Therefore, pre-treatment is necessary to break down cell walls to free intracellular organics. The solubilized cell components are much more biodegradable; thus, the HRT of the AD process is reduced and the efficiency is improved when they are released [48].

To improve AD efficiency and accelerate the rate-limiting hydrolysis, various pre-treatment technologies including advanced oxidation processes, chemical and physical, have been developed [11]. Most methods are energy-intensive, and thus, their application requires careful economic feasibility analysis [48]. Anaerobic co-digestion (AcD) of FW with other waste streams has gained attention as an effective waste management treatment method for WAS because it can counter-balance nutrient deficiency, promote economic feasibility, and introduce a waste management solution to more than one waste stream. Moreover, the high acidity of FW makes its mono-digestion unfavourable.

Anaerobic codigestion in a WWTP is the addition of organic-rich waste materials to the sludge during digestion for improved efficiency [49]. The AcD offers one of the most important advantages of having the potential to increase the efficiency of organic waste degradation and thus methane production. A case study on the benefits and disadvantages of the codigestion of food and dairy waste at high organic loading rates with sludge sewage was conducted by Sembera et al. [50]. They found that the potential increase of methane was 300%. Co-substrates are positively synergised in the digestive medium, thus serving as a source of lost nutrients. The yield of biogas from mono-digestion to AcD is increased [49]. Additionally, the diverted waste disposal route reduces the need for more landfill space and potentially aids municipalities in achieving their waste management goals.

WWTPs and municipalities are now expected to have cost savings from the use of wastes integrated into their existing anaerobic mono-digestion systems. With increased biogas production, there is a reduction in the plant's grid energy costs resulting from excess generation of electricity at the site, which can be sold and fed into the national grid; this upgrades the WWTP to a power supplier as well. Sembera et al. [50] report that from 2014 to 2015, at the Moosburg WWTP, there was an increase in hourly electricity production because of increased methane yield, which resulted in energy neutrality. Further economic gains were from gate fees. The co-substrates can supply micronutrients and alkalinity, overcoming limitations of mono-digestion of each substrate, resulting in more efficient use of equipment by improving process performance [44].

Although AcD studies have shown that it has advantages over mono-digestion and its effective result in treating waste, there is limited research comparing the feasibility of various co-substrates due to their influential factors on digestion and best operating conditions [49]. Also, although AcD offers a lot of advantages, many researchers continue to be plagued by problems in their performance; this often leads to system malfunction largely as a result of an inappropriate ratio of substrates and operating conditions [51], [52]. The disadvantages that have been seen by Sembera et al. [50] are that solids concentration tends to increase within the digester, higher nitrogen backloading, lower retention time and reduced digestion efficiency. Consequently, a good understanding of AcD is required for a successful outcome. Previous research in Table 7 shows varying applications and improvements of AcD of wastewater sludge with organic-rich FW and organic fraction of municipal solid wastes (OFMSW) at mesophilic temperatures.

Improvements in methane yield (MY) as low as 16% and as high as 122% may be obtained from Cabbai et al. [53] and Cavinato et al. [54], respectively. Cavinato et al. [54] co-digested WAS with OFMSW at a 50:50 mixing ratio by substrate volume at an OLR of 1.6 gVS/L/day and obtained 0.09 LCH₄/gVSadded for monodigestion and 0.2 LCH₄/gVSadded for co-digestion. Cabbai et al. [53] digested SS with FW at a ratio of 1:0.23 as per VS obtained higher MYs than Cavinato et al. [54] of 0.25 LCH4/gVSadded during mono-digestion and 0.29 LCH4/gVSadded for AcD. It was found that when compared with lower amounts of FW, higher concentrations of FW were responsible for increasing the methane yield in the substrate mixture. In particular, FW contains a higher amount of biodegradable volatile organic matter for AD compared to sludge, which is quite resistant to hydrolysis [55]. Various other factors, such as reactor size, contaminants in OFMSW, and OLR, affect the throughput of the process, as seen in Table 7.

Various types of digesters are used to treat wastewater sludge and food waste; however, they all perform the same basic function. Digesters of all types hold organic substrate in the absence of oxygen and facilitate suitable conditions for the growth of methanogens [60]. Digester applicability is based on the technical suitability, cost-effectiveness and availability of local skills and materials. Though a specific design may have proven successful for treating a range of feedstock, it still depends on the prevailing climatic and economic environment the installer faces. Conventionally, wastewater treatment plants in South Africa apply a variety of layouts for the complete-mix/fixed-dome type digester for sludge stabilisation, as depicted in Figure 2. This type of digester provides excellent mixing and heated conditions, moreover, a cylindrical tank with a conical top and bottom is recommended [61]. Due to the heating and mixing provided, the digester enables operation at higher total solids of 3 to 10%, allowing for organic loadings of 1.5 to 3 kgVS/m³/day [62] and reduced retention times from 50 to 80 days to 20 to 30 days [61]. Digester heating may be carried out using external or internal heat exchange coils heating water or steam in a boiler. Mixing may be through draft tubes (internal or external), gas-mixing, mechanical mixing, or jet- or nozzle-mixing [63].

The high-rate characteristic of the complete-mix digester makes it equally suitable for the digestion of FW. All 420 digesters at 108 WWTPs in South Africa use a variation of the complete-mix digester [63]. Thus, the municipal WWTP digesters are already equipped for introducing food waste as a co-substrate in that regard [65]. Additional equipment is, however, required for food waste handling before digestion and power generation.

WWTPs are equipped for the mono-digestion of sludge. To enable anaerobic co-digestion with FW, a preliminary FW pre-treatment unit is required, known as the materials recovery facility (MRF) depicted in Figure 3. The MRF facilitates sorting organic waste from foreign objects and the crushing of whole foods into smaller particle sizes that can be easily fed to the anaerobic digesters at a total solids concentration acceptable by the existing pumps and downstream equipment. Furthermore, conventional WWTPs in South Africa are not designed to make use of the produced biogas except for digester heating through water boilers, where excess biogas is flared [63]. In that regard, equipment is required to manage biogas conversion to electricity and heat.

The conversion of biogas into electricity is carried out by internal combustion engines known as combined heat and power (CHP) units that are paired with a set of biogas scrubbers employed to remove any impurities hazardous to the generators' internal components, such as sulfur and condensation (Figure 4). The requirements for Genset, a typical generator used, are listed in Table 9 [66]. In this large-scale application, the CHP units are used to convert biogas into electricity and heat. Heat can be mainly used to heat the anaerobic digesters to mesophilic and even thermophilic temperatures, while excess heat and power may be sold to the public [71].

South Africa produces 12.6 million tons of FW per year [25]. This amount of FW can yield a significant amount of biogas. The biogas potential of this food waste can be calculated using eq. (1) and (2). An anaerobic digester treating FW and operated at mesophilic optimal conditions obtained a biogas yield (BY) of 879 L/kgVS_added, as reported by Xu et al. [44]. In the 12.6 million tons of FW, volatile solids VS is assumed to be 95% and TS 15%; thus, the total VS (TVS) available would be 1 850 000 000 kgVS. The total biogas production potential is 1 630 000 m³.

Biogas can be converted into electricity through small and large combustion engines. Larger engines have a combined heat and power (CHP) capacity to provide heat to the anaerobic digester and other processes. Typically, small engines have a conversion efficiency of 25% and 35 – 40% for larger engines [68].

This analysis uses an engine conversion efficiency of 25 and 40% to predict energy potential for small and large-scale electricity generation, respectively. Equation (3) with the conversion efficiencies may be used to calculate the energy potential from the biogas:

where e_biogas is the total electricity in kWh generated from biogas, E_biogas is the available raw biogas in m³, CVis the calorific value 22 MJ/ m³ of biogas at 60% methane, 0.277778 kWh/MJ is a unit conversion from MJ to kW and the overall conversion efficiency of the generator is represented by the symbol η [72].

Therefore, the annual biogas electricity potential of South Africa based on their FW amounts is 3 485 914 025 975 kW, which is 3% of the target for new electricity production from renewable energy to contribute to the national 227 TWh energy mix which is set by the Integrated Resource Plan of South Africa and initiated by the Department of Energy [73].

One of the first commercial anaerobic digesters to produce biogas and generate electricity in South Africa was John Fry, who built it in 1957 at his pig farm [74] (Figure 5). Currently, there are about 300 different-sized operating biogas plants in South Africa. Johannesburg Waters (JW), a municipality based in Gauteng province, has four working anaerobic digesters to reduce the total organic load on their wastewater treatment plant and stabilise the sludge before disposal (Figure 6b). JW is producing approximately 2 million m³/yr of biogas from wastewater sludge. WEC Projects uses biogas produced from JW WWTP and runs a set of three generators, each with a power output of 300 kW and instantly feeds approximately 5 MW of electricity annually to the grid.

Bio2watt in Bronkhortspruit, South Africa, runs a 4.6 MW biogas power plant from about 120,000 tons of organic waste from a Beef Farm per year. Companies such as Botala Energy Solutions and Biogas SA can be hired to install small- to large-scale plants around South Africa. One of Botala’s biogas plants depicted in Figure 6a has a 100 kV combined heat and power plant for a local village in Tshwane with fresh produce as digester feedstock. The community benefits from the electricity and fertiliser produced from their fresh produce waste material. Nationally with only 108 WWTPs out of 824 utilising AD for sludge stabilisation, there is an annual biogas production potential of 103 174 915 m³ and 240 084 225 kWh of electricity from 1 291 735 ML of sludge [63].

Even though biogas in South Africa was first produced in the 1950s [74], [75] its use remains very low. According to Mukumba, Makaka and Mamphweli [75], this is due to the lack of research work on biogas technology and purification processes, which leads to the low efficiency of biogas compared to conventional fuels such as diesel and petrol. Other factors contributing to the low uptake of AD technology include low electricity costs from coal-fired thermal power stations, education, awareness of biogas in general, and funding for establishing and maintaining biogas digesters. The lack of a generic solution to run a digester was one of the main challenges reported on biogas development in South Africa. Most of the data available are based on research in other countries and cannot be used directly in South Africa. Thus, more South African anaerobic digestion research needs to be conducted to promote biogas utilisation locally [75].