Introduction

Restaurants and domestic household discharging wastewaters into public sewers have been a problem for many years, and it becomes greater with a large number of full-service and fast-food restaurants being built both in large cities and rural communities (^{Shamsuddin et al., 2020}, ^{Dumore & Mukhopadhyay 2012}). These restaurants typically discharge a large amount of fat, oil, and grease (FOG) that would reduce the capacity of public sewers over time ^{(Shamsuddin et al., 2020}). FOG is a type of waste proceeding from the production of food (^{Gibbons, O´Dwyer & Curran, 2015}). Fats and oils are among the main components of organic matter in wastewater (^{Ruggieri et al., 2008}; ^{Saatci et al 2001}). Solid wastes, especially those produced by the food industry ^{(Ruggieri et al., 2008}, ^{Galli et al., 1997}; ^{Mari et al., 2003}) are fats and oils essentially composed of triglycerides consisting of linear fatty acids attached, as esters, to glycerol ^{(Ruggieri et al., 2008}, ^{Lalman et al., 2000}). FOG is an ever-growing environmental concern. This waste is usually produced at food service establishments (FSE) or other food preparation facilities. The by-products and wastes from these FSE include meat, sauces, gravy, dressings, deep-fried food, baked goods, cheeses, butter ^{(Hussain et al., 2014}), food scraps, meat fats, lard, tallow, cooking oil, butter, margarine, sauces, gravy, dressings, deep-fried food, baked goods, cheeses, and butter (^{Hussain et al., 2014}; ^{Aziz et al., 2011}).

Currently, there is only one method to control FOG at the source of generation. This method involves installing a grease interceptor (GI) that separates the FOG from the restaurants´ sludge, reducing its concentration in the effluent (^{Hussain et al., 2014}; ^{Aziz et al., 2011}).

The FOG is collected manually or automatically, after its separation in grease traps, and then, it is recycled into bio-diesel or open dumped with solid waste (^{Almeida et al., 2017}; ^{Canakci, 2007}). The residual fat material from grease trap, a lipid-base material of low quality, consists of fatty acids, frying oils (soybean y sunflower), animal fats, hydrogenated fats, fatty alcohols, and other compounds (^{He et al., 2011}; ^{Ruggierei et al., 2008}). Therefore, FOG components are introduced into the sewer system either by direct dumping into the sewer or by escape from grease traps (GTs) (^{Hussain et al., 2014}). FOG may solidify and form particles that deposit on the surface of the pipe, thus obstructing the wastewater flow (^{Hussain et al., 2014}, ^{Prakash et al., 2015}; ^{De-qing et al., 2007}). FOG blockage is a worldwide concern. For example, the American Environmental Agency (EPA) estimated that at least 10.350 to 36.000 sanitary sewer overflows (SSO) occur per year in the USA, approximately 47 ％ of is related to FOG (^{Hussain et al., 2014}; ^{Idris and Ahmed, 2003}). The block- ages and sewer flooding may result in other environmental problems, both locally and beyond the premises (^{Hussain et al., 2014}).

Fat oil and grease tend to stick to the surface of drain and sewer pipes causing clogging that restricts the flow of sewage and may lead to sanitary sewer overflows (SSO). SSOs cause unpleasant odors and insect and rat infestation, and the sewage may make its way into water sources causing ground and surface water pollution (^{Hussain et al., 2014}; ^{Sasaki et al., 2003}).

Recently, extensive effort has been expended to investigate the possibility of treating and reusing FOG to reduce the amount disposed at landfills ^{(Hussain et al., 2014}). Among the available technologies to recycle organic solid wastes, composting is often presented as a low-technology and low-investment process to convert organic solid wastes to a soil amendment known as compost (^{Vicencio de la Rosa, Valencia & Ortega, 2013}). Composting is a biotechnological process by which different microbial communities initially biodegrade organic matter into simpler nutrients and, in a second stage, form complex organic macromolecules, such as humic acids ^{(Vicencio de la Rosa et al., 2013}; ^{Roman et al., 2013}). Composting offers an economical and effective way to treat oil sludge. Composting process which involves the careful control and addition of nutrients, watering, tilling, in addition to suitable microflora and bulking agents (wood-chips or hay) were considered as an alternative option to improve the bioremediation of oil sludge (^{Torres et al., 2005}; ^{Albino, 2017}).

The main advantage of composting is waste stabilization (^{Torres et al., 2005}; ^{Robles et al., 2017}). Composting matrices and composts are rich sources of xenobiotic degrading microorganisms including bacteria, actinomycetes and lignolytic fungi, which can degrade pollutants into compounds such as carbon dioxide and water. These microorganisms can also bio transform pollutants into less toxic substances and/or lock up pollutants within the organic matrix, thereby reducing pollutant bioavailability ^{(Torres et al., 2005}; ^{Semple et al., 2001}).

Composting of fats, oil and grease is inherently difficult due to their nutrients deficiency, with especially low nitrogen and phosphorous content relative to high carbon content (^{Ruggieri et al., 2008}; ^{Sasaki et al., 2003}). This fact usually implies the use of a co-substrate to compensate the C/N ratio of the initial mixture and to act as inoculum. Different types of sludge, due to their typical low C/N ratio, are considered suitable for being composted with FOG or FOG-enriched wastes (^{Ruggieri et al., 2008}; ^{Wakelin and Forster 1997}).

Therefore, it is intended to evaluate the residues of the grease traps of the sludge from the sodas of the National University by means of an enzymatic composting technique for its later use as an amendment.

Methodology

Analysis and results

Table 1 summarizes the average of population served as well as the average sludge production (kg/month) for each FSE during one-year. A total of 21.023 people attended the different food service establishments. The minimum quantity of visitors corresponds to FSE-3, with a percentage of 16.76 %. On the other hand, the places with a greater number of users are FSE -2 and FSE -4 with a total of 6778 (32.24 %) and 6543 (31.12 %), respectively.

According to the table below, the minor sludge generation rate (15.97 ± 5.56 kg/month) corresponds to the FSE- 4. This low generation is explained because the lack of a grease and oil trap system, and good cleaning practices during the study period; therefore, limited quantity of sludge was retained. The FSE-4 meal menu offer is another reason for this low rate, in this place fried foods are less included in the menu compared to the rest of establishments evaluated. Contrary to the mayor sludge rate generation is for FSE-2 (53.19 Kg/month, 45.20 %), this value is due to the largest population served by this place, and also because of the high content of fatty food on the meal menu. However, according to this research, the main reason of the biggest sludge generation is related to the population served.

Table 2 indicates the sludge and composting average amounts obtained from the food service establishments. A monthly average of 118.56 kg of sludge was collected and then deposited during 30 days in drying beds. After the drying process, this amount was reduced to 55.10 kg/month. This reduction obeys to the loss of humidity. The dried sludge and 12.16 kg of co-substrate (sawdust pellets) were deposited on a food waste composter, both products were constantly mixed to produce the composting. The total monthly composting amount is 39.35 kg.

Figure 1 shows the results of pH values, the concentration (mg/L) of organic matter (OM), carbon (C), nitrogen (N) as well as the carbon and nitrogen relation (C/N) and carbon and phosphorous relation (C/P) for the sludge coming from the drying bed (SDB), the non-enzymatic composting (NEC) and the enzymatic composting (EC). Regarding the concentration of nitrogen, carbon and organic matter, figure 1 shows an organic matter concentration greater than N and C concentrations for the three substrates. On the other hand, nitrogen concentration is the lowest value reported for the substrates. When comparing C/N and C/P relations on each substrate, the higher value corresponds to the C/P relation.

The results obtained when comparing substrates to one each other, indicates a nitrogen percentage for the sludge from the drying bed (SDB) major than the reported for the non-enzymatic composting (NEC) and enzymatic composting (EC). The same pattern is for the percentage of carbon. However, the individual content of carbon and nitrogen are not determinants for the quality of the composting. Nevertheless, it is important to keep an adequate C/N relation, ^{Melendez & Soto (2003}) recommend a relation less than 25. According to the data gathered (for the three studied substrates), the C/N relation values are over this standard. This situation obeys to the low nitrogen content in the oil and greases coming from the trap system, the sludge collected is a poor nitrogen source. In contrast, the original sludge shows a high content of carbon, due to the large and stable carbon molecules existing in the grease and oils previously collected. The low content of carbon could also be explained because of the low mineralization activity during the drying bed process.

Related to the composting samples, figure 1 shows a C/N relation inferior to the enzymatic composting (EC) when being compared to the non-enzymatic composting (NEC). The grease and oil degradation process are more efficient in the EC. This could be associated to the activity of the enzymes, which degrade more carbon, therefore decrease the C/N relation. The C/N relation could also be improved by using substrates with high content of nitrogen; for example, urea, fruit peels, animal excreta and grass, increasing thus, the nitrogen concentration (^{Vicencio de la Rosa et al., 2013}).

Figure 1 Results of the chemical basic elements for the substrates. (2012). 

Related to the C/P relation, ^{Melendez & Soto (2003}) point it out as a main aspect to improve the mineralization process. These authors recommend a value minor than 200. However, the data obtained for all substrates is over this standard, 272.46, 203.14 and 228.08 for the SDB, EC and the NEC, respectively. This tendency is due to the same explanation discussed for the C/N relation, especially about the high carbon content existing in the original sludge. It is important to point that regarding to the individual content of phosphorus, the substrates evaluated meets the reference parameter recommended by ^{Roman et al., (2013}), which is between 0.1 % and 0.4 %. The %N are 0.20 %, 0.25 % and 0.22 % for the SDB, EC and NEC, respectively (Figure 3).

According to the organic matter content, (Figure 1), the three substrates show an organic matter percentage superior to 80%. ^{Roman, Martínez & Pantoja (2013}) suggest a value in the range of 50 % and 70 %. The excess of OM could be explained due the presence of long and stable carbons chain molecules in the original sludge. Nevertheless, it was expected to obtain an organic matter percentage between 50 % and 70 % for the enzymatic composting; however, the results show similar values for the 3 substrates (89.40-86.48 and 85. 33 for the SDB, EC and NEC, respectively). This similitude indicates a weak organic biodegradation matter activity by the enzymes in the enzymatic composting. It is recommended to extent the enzymatic process time to have a better mineralization of the organic matter by the microorganisms. Despite the content of organic matter is high for the 3 studied substrates ^{(Roman et al., 2013}), some other authors indicate that biosolids with an organic matter percentage over 60 are very efficient to use on bioremediation process to recover heavy metals polluted soils (^{Vicencio de la Rosa et al., 2013}; ^{Torres et al., 2005}). Therefore, substrates with OM content superior to 60% decrease through adsorption process of the heavy metals bioavailability of the soils, leading them, the formation of a stable organic metallic complex. According to this, the evaluated substrates (SDB, EC and NEC) could be used as an improving and stabilizing agent to treat metal contaminated soils.

The biggest values of Zn and Cu were found in the sludge, while the lowest values were found for the non-enzymatic composting. The difference between the micronutrient concentrations is because the substrate named SDB has not experienced through biological activity. Therefore, there is a lack of microorganisms to consume the micronutrients, hence, this substrate has a major bioavailability of zinc and copper.

Considering the enzyme activity, it was expected to obtain an inferior micronutrient concentration in the EC substrate when comparing with the NEC substrate. Nonetheless the results show (Figure 2) Cu and Zn concentrations smaller for the NEC substrate. This is due, the non-enzymatic composting substrate contain greater quantity of micro microorganisms than the EC substrate. Therefore, the NEC substrate requires high number of micronutrients, in this case, Zinc and Copper. In the case of the EC substrate, the activity of the enzymes accelerates the substrate biodegradation, hence, there is a superior micronutrients availability in the enzymatic composting compared to the one without enzymes. To consider a substrate non-toxic for the improving and the cultivation of plants and microorganism, this, must have concentrations of Cu and Zinc less than 100 mg/kg and 200 mg/kg respectively (^{Albino, 2017}). In this study, the three substrates (SDB, NEC and EC) show lower values than the one reports by ^{Albino (2017}). Thus, these substrates could be considered a non-toxic substrate. For the sludge coming from the drying bed, the concentration (mg/L) of Zinc and Cupper are 71.43 and 19.13 respectively, for the enzymatic composting are 46.29 and 13.53 respectively, and for the non-enzymatic composting are 39.43 mg/L Zn and 10.53 mg/L Cu.

Figure 2 Microelement concentrations for the substrates; sludge drying bed, enzymatic composting and non-enzymatic composting. (2012). 

Figure 3 shows the total phosphorus (P₂O₅), potassium (K), magnesium (Mg) and calcium (Ca) percentage obtained for the 3 evaluated substrates. The sludge substrate has the lowest concentrations (mg/L) for P (0.20), K (0.11), Mg (0.11) and Ca (5.93). These concentrations are principally linked with the sludge higher acidity (pH = 5.15) in comparison with the given pH for the NEC (6.29) and EC (pH 6.29). Acid pH values lead to a low magnesium and potassium concentrations ^{(Robles, J et al., 2017}). Contrary, the highest levels (mg/L) of Ca (6.17), Mg (0.19), K (0.70) and P₂O₅ (0.25) were found in the enzymatic composting, which also have the highest pH value. The enzymatic composting basic pH value allow a high macronutrients availability in comparison with the other two substrates (the sludge and the non-enzymatic composting). For the 3 substrates, calcium shows the higher concentration (Figure 3). The main reason for this situation, is the calcium oxide used to level the sludge previously collected from the trap systems. ^{Albino (2017}) suggest a calcium concentration between 20 and 65 % for a substrate that will be used as an organic fertilizer. The calcium concentration found for the 3 analyzed substrates were less than this reference. The main reason for this calcium deficiency obeys to the low content of calcium included on the original substrate (sludge from the oil and grease trap). Nevertheless, considering its low cost, calcium could be added after the composting process.

Figure 3 Total phosphorus (P ₂ O ₅ ), potassium (KO), magnesium (Mg) and calcium percentage for the sludge coming from drying bed, the enzymatic composting and non-enzymatic composting. (2012). 

In relation to the phosphorus concentration, ^{Melendez & Soto (2003}), recommend a concentration range in an organic fertilizer between 0.15 - 1.5 %. Data from the above figure indicates a P₂O₅ concentration within this range for the 3 samples studied (0.20-0.25 and 0.22 % for the SDB, EC and NEC respectively). In the case of potassium concentration for an organic fertilizer, ^{Roman et al., (2013}) recommend a content between 0.3 -1.0 %. For this research, both composting substrates have values between this parameter, 0.70 % and 0.65 % for EC and NEC, respectively. However, some other authors (64) have found phosphorus concentrations over the values recommended by ^{Roman et al., (2013}), especially in biosolid with high content of grease and from sludge coming from sewage treatment systems. Table 3 shows the heavy metal concentrations for the 3 evaluated substrates. The analyzed metals were cadmium (Cd), Nickel (Ni), Mercury (Hg) and Lead (Pb). One Costa Rican regulation and seven international regulations were used to analyze the gathered results. The regulations are from the following countries: Costa Rica, Chile, Mexico, Germany, Great Britain, Switzerland, France, and Canada.

The sludge from the drying bed (SDB) was evaluated using 3 regulations (Costa Rica, Chile and Mexico). In the case of the non-enzymatic composting (NEC) and the enzymatic composting (EC) six of the seven national regulations were used (The only regulation excluded was the Mexican Law). The Costa Rican and Chilean regulations are focused in controlling and regulating the heavy metals permissible limits on the bio solids and in the composting used as a substrate to enrich organic fertilizer used in plant crops. The approach of the Chilean and Canadian normative exclusively regulates the content of heavy metals in the composting. In the case of the sludge, the heavy metal concentrations are within the range established by the 3 regulations used as reference. Related to the composting (EC and NEC), cadmium (1.73 mg/kg) is the only heavy metal with a concentration over the Swiss (Cd < 1) regulation permissible limit. However, for the three substrates, the cadmium concentrations, as well as the other metal concentrations, meet the standard reference value recommended in the rest of the regulations. A substrate used as an organic fertilizer containing high heavy metal concentrations is prejudicial to plant crops growth, to soil microorganisms, to the animal health and to the human being. According to ^{Albino (2017}), the sludge and composting substrates obtained for this research could be applied as organic fertilizer, considering its low heavy metal concentrations. During this investigation it was not possible to correlate the heavy metal concentrations (HMC) with the enzymatic activity. The data obtained shows a linear behavior, because there is no exponential increase or decrease in metal concentrations between one sample and another.

Conclusions

Good cleaning practices and a menu with less frying influence that FSE-4 generates less sludge than FSE-2, even though both serve a similar amount of population.

The average amount of sludge collected was reduced to approximately half its original weight after the drying stage. This reduction of the water content prevents the anaerobic decomposition of the sludge, thus improving the composting process.

The collected sludge is not a good energy alternative, since it does not reach the required temperature for this type of exploitation. CE and NEC could be like organic fertilizer. The oil and fat collected are not a good source of nitrogen, due to their high C / N ratio. The three substrates have a high content of organic matter which could be treated in bioremediation processes in soils contaminated with heavy metals. Ca is below the standard level. The Zn and Cu values were found in the sludge, while the lowest values were found higher for non-enzymatic composting.

For future research, some recommendations are: