Assessment of two sludge stabilization methods in a wastewater treatment plant in Sotaquirá, Colombia

Wastewater sludge is a by-product of wastewater treatment; it is often employed in agricultural processes following appropriate microbiological stabilization. In this study, we evaluated two methods of sludge stabilization in the wastewater treatment plant of Sotaquirá, Colombia. The two assessed stabilization methods were dewatering on drying beds and alkaline stabilization. The stabilization through dewatering on drying bed was carried out for five months. Alkaline stabilization was conducted for 96 hours with three concentrations of calcium oxide, 7 %, 9 %, and 13 % w/w. Humidity, pH, organic carbon, phosphorus, organic nitrogen, fecal coliforms, somatic phages, total helminth eggs, and Salmonella sp. were monitored monthly for the sludge under dewatering treatment and at 0, 12, 24 hours for sludge under alkaline stabilization treatment. Both treatments succeded in eliminating Salmonella sp. Helminth eggs were eliminated by alkaline stabilization, whereas it was reduced to one or zero helminth eggs with the dewatering treatment. Somatic phages were eliminated with alkaline stabilization but were only reduced to 3.52 log CFU/g with the dewatering method. Dewatering on drying beds produced biosolids that can be used for soil restoration. Whereas alkaline stabilization produced biosolids that can be used for agricultural purposes. Alkaline stabilization with 9 % and 13 % calcium oxide ostensibly reduced nitrogen and phosphorus contents in the sludge, whereas 7 % calcium affected less the phosphorus concentration of the sludge. These results indicate that sludge dewatering on drying beds is an effective sludge sanitation protocol to be in implemented in small wastewater treatment plants, such that in Sotaquirá, Colombia.

In this regard, Colombian regulations deem sewage sludge stabilization as a way to reduce pathogens and further classify its biosolids, based on their physicochemical and microbiological properties, into categories A and B (see Table 1 and Table 3). Wastewater sludge type A biosolids can be used for agricultural purposes without restrictions. Whereas, sewage sludge type B biosolids can only be used for soil restoration [4].
In Colombia, 97 % of the sludge produced by wastewater treatment plants (WWTP) is generated in the three major urban areas, Bogotá, Cali, and Medellín [7]. The remaining 3 % is produced by several small municipal plants across the country. In these municipal WWTPs knowledge about how to appropriately manage growing sewage sludge is missing. Usually, these WWTPs dispose the sludge on nearby fields or give it away to local farmers without any treatment, oblivious of the negative impact of their practices on the environment and the public health.
The wastewater treatment plant in the locality of Sotaquirá, (Department of Boyacá) Colombia, produces 109 tons of total solids per year. The sewage sludge generated by this plant is disposed on sludge drying beds for a short period of time, usually less than two months, due to its limited storage capacity. Then, it is given away to local farmers without any further treatment. This is seen as a win-win situation for the plant managers and the farmers because the plant gets rid of the by-product and the farmers use it to increase the productivity of their crops. However, they do not realize the negative health and environmental consequences that this practice may cause. This sewage sludge exceeds the pathogenic microorganism concentrations permitted for agricultural purposes or soil restoration [4]. This problem exists even though there are several stabilization techniques that have been proven to successfully reduce the pathogenic bacteria concentration to the required levels by local regulations.

Materials and methods
Description of the area of study The wastewater treatment plant is located in Sotaquirá, in the department of Boyacá, Colombia (5 • 45' 4" N, 73 • 14' 50.9" W) at 2 860 meters above sea level. The average annual temperature of this locality is 14 • C with an average annual relative humidity of 80 %, an average annual precipitation of 1 260 mm, and an average wind speed of 21 Km/h. The main economic activity of this town is dairy production and all its discharges flow into the sewage system. The wastewater treatment plant treats an average flow of 1.72 L/s.

Collection of samples
The wastewater sludge accumulated in an Imhoff tank is uniformly distributed on one of the two sludge drying beds available. The sludge drying bed used in this study has an effective area of 5.5 m 2 . A transparent plastic sheet was used to block rainfall without blocking solar radiation. The drying process consisted of sun exposure for five months. During this period, dewatering sludge was sampled monthly from the surface of the bed. Each sample was taken by digging small holes from several points until collecting approximately 1 Kg. The monthly samples were collected and stored in a hermetic storage bag at 4 • C for 48 h before analysis.

Laboratory analyses
Physicochemical and microbiological analyses were carried out on the collected samples. Prior to analysis, the samples were pulverized and sieved through a 1.18 mm stainless steel mesh (Endecotts, United Kingdom). As for the microbiological analysis, fecal coliforms, helminth eggs, Salmonella sp., and somatic phages were assesed. Whereas in the physicochemical analysis, heavy metals, total organic carbon (TOC), total phosphorus, organic nitrogen, pH, humidity, conductivity, and organic matter were quantified. Heavy metal analyses were carried out only with the sample collected at the beginning of the study. Most of the other analyses were done in triplicate every month for a five-month period. The methods used for each of the analyses are listed in Table 2.
A sample of 100 g of WWS was collected after two months of natural dewatering in drying sludge beds. This sample was pulverized, weighed, and thouroghly mixed with reagent grade calcium oxide (CaO) in the following three concentrations 7 %, 9 %, and 13 %, on a dry weight basis (w/w). The three sample mixtures were analyzed at 0, 12, 24, and 96 hours for total organic carbon concentration, organic matter load, total phosphorus concentration, pH, and conductivity. Total coliforms, total helminth eggs, Salmonella sp., and somatic phages were also assesed. All data from physical-chemical variable quantifications and microbiological test results were handled and analyzed using R [17].

Results and discussion
The concentration of heavy metals in the samples of wastewater sludge from Sotaquira's WWTP are shown in Table 3. The obtained concentrations of these metals are below permited values for agricultural use without restrictions, thus this sludge can be safely assigned to category A. These results may be related to the absence of industrial and mining activity in the area of the municipality. In comparison, sludge in big cities with high industrial development presents higher values of heavy metals. For instance, readings of the WWTP of Medellin have revealed a copper in a concetration of 894.6 mg/Kg, cadmium 11.17 mg/Kg, lead 94.6 mg/Kg, and nickel 398.2 mg/Kg [18].
Physicochemical and microbiological features of the sludge on drying bed treatment The sludge discharged onto the drying bed had a pH value of 5.5. This pH value could be explained by fermentation of organic matter by facultative anaerobic bacteria. After one month on the drying bed, the pH values increased and remained around 6.84 (± 0.20) for the rest of the drying period. These results differ from the results obtained by Cota-Espericueta and Ponce-Corral using a solar dryer [19]. They observed an initial pH level of 6.8. After 74 hours, the pH decreased to 6.07 reaching a level of 6.0 by the end of the eleven-day treatment.
The TOC in the sludge exiting the water treatment plant was 44.00 % (± 0.17).
During the five-month drying period, the content of organic carbon decreased steadily, reaching 37.3 % (± 1.53) (Fig. 1 The temporal profile for total phosphorus is displayed in Fig. 2. Before the second month, total phosphorus remained around 5.21 % (± 0.15). In the second month, the average concentration was reduced to 4.73 % (± 0.05).
From the second to the fifth months, the concentration increased steadily until reaching a concentration of 5.08 %(± 0.06). Regarding organic nitrogen, its values fluctuated around 1.91 % (± 0.32). The total phosphorus and organic nitrogen values measured at this plant were higher than those reported by other domestic wastewater treatment plants [11, [21][22][23]. These high levels are attributable to the main local economic activity of the municipality, which is the production of artisanal dairy products.   Microbiological analyses showed that the five-month sludge drying process on drying bed had a significant influence on the pathogenic microorganisms. By the fourth month of drying, Salmonella sp., was not detected. This absence could be justified by the fact that Salmonella sp. is a microorganism sensitive to desiccation and is often outcopided by other microorganisms [24,25].

Metal
Fecal coliforms decreased from 10.9 log CFU/g (± 0.07) to 4.11 log CFU/g (± 0.08) during this study. Whith the first two months, fecal coliforms decreased more rapidly than in the third to the fifth months (Fig. 3). The pattern of decrease of fecal coliforms concentrations matches that observed for humidity. Humidity was reduced by 88.9 % at the end of the second month (Fig. 3); subsequently, it remained relatively constant and reached its lowest point (< 6 %) by the end of the fifth month. Fig. 4 displays the number of total helminth eggs found in the monthly samples. A high prevalence of Ascaris lumbricoides (57 %) was observed, followed by Fasciola sp., Diphyllobothrium sp., Clonorchis sp., Schistosoma spp. and Hookworm sp. The mean number of total helminth eggs and its variability decreased with time. An average of 9 (± 7.55) total helminth eggs were found in the sludge sample leaving the treatment plant. After five months on the drying bed, only one egg was found on one of the three samples.  An average of 4.36 (± 0.312) log CFU/g somatic phages were observed in the sludge exiting the treatment plant. After two months on drying beds, the log CFU/g somatic phages decreased to 3.52 (± 0.03). However, after this initial decrease, there was no change in the somatic phages content. These results are consistent with those reported by Shanahan and Campos [24][25][26].

Physicochemical and microbiological features of sludge under alkaline stabilization
The application of CaO at different doses (7 %, 9 %, and 13 %) produced an increase in the pH of the sludge. The pH of the sludge treated with the highest concentration of CaO (13 %) increased to 12 and remained at this level for more than 96 hours (Fig. 5). This treatment complies with one of the alternatives given by the Environmental Protection Agency (EPA) and Colombian regulations to obtain type A sewage sludge [4]. In contrast, the average sludge pHs achieved with CaO concentrations of 9 % and 7 % were 11.65 and 11.09, respectively.  Alcaline treatment with 13 % CaO led to an immediate total reduction of fecal coliforms, Salmonella sp., and somatic phages. In the case of helminth eggs, no eggs were detected at time 0 and after 12 hours, but one helminth egg was detected in one of the three samples 24 hours after treatment. No helminth eggs were detected 96 hours after treatment. Alcaline treatment with CaO at concentrations of 9 % and 7 % eliminated the fecal contamination indicators after 24 hours and 4 days of exposure, respectively. This elimination is due to the increase in temperature and pH for two or more hours with release of gases and hindering of pathogen reproduction [27]. Both of these concentrations also eliminated Salmonella sp., right after application. Helminth eggs were also eliminated with these two CaO concentrations. The application of 9 % CaO resulted in the elimination of all helminth eggs after 12 hours, whereas 7 % CaO elimanted all eggs at 24 hours. Somatic phages were eliminated after 12 hours with both concentrations.
Sludge treatment with 13 % CaO led to results similar to those obtained by Torres et al [28] with 15 % CaO, observed helminth egg and fecal coliforms elimination at time zero. These authors also used 8 % CaO and found that helminth eggs were also eliminated at time zero, whereas fecal coliforms were eliminated by the sixth day after application. Silva-Leal et al obtained comparable results for helminth eggs with CaO at concentrations of 8 %, 8.5 %, 9.0 %, 9.5 %, and 10 % [29]. Furthermore, contrary to the results reported by Torres et al [28] and the present study, Silva-Leal and collegaues did not detect coliforms at the time of CaO application, regarless of its concentration. Another difference to the present study is that they reached a pH of 12 with all tested CaO concentrations for more than 72 hours as required by EPA.
The treatment with 7 % CaO resulted in sludge phosphorus concentrations of 4.6 % and 4.3 % at the time of application and at 24 h after application, respectively. These concentrations are slightly lower than the ones obtained without alkaline stabilization. On the other hand, in the mixtures treated with 9 % and 13 % CaO phospohorus concentrations at 24 h after application were of 1.0 % and 0.5 %, respectively. The concentrations of organic nitrogen in the sludge mixtures treated with 13 %, 9 %, and 7 % CaO were 1.2 %, 0.9 %, and 1.1 %, respectively. These values are all lower than the obtained without alkaline stabilization.
Based on the physicochemical analysis and microbiological assessment results, we were able to ascertain that sludge dewatering on drying beds is an effective treatment to reduce fecal pollution indicators without altering the sludge's physicochemical properties of agricultural importance. In this study, the biosolid obtained using drying beds was of category B according to Colombian regulations, failing to meet criteria to be considered as a category A biosolid because of reported fecal coliform concentrarions. Given that sludge dewatering on drying beds is a process where sunlight and wind stabilize the sludge, it is regarded as a low-cost process with operational simplicity and limited energy consumption. Thus, it is a viable alternative to treat the sludge produced by small WWTPs. However, it requires space and infrastructure that several small plants may not have. On the other hand, alkaline stabilization requires less time and less space; however, the treatment of all the sludge produced will require large quantities of CaO. Therefore, it is desirable to use the minimal concentration of CaO that can reduce fecal pollution indicators. For the small WWTP under study, alkaline stabilization with 7 % CaO can yield A category biosolids.

Conclusions
The physicochemical characterization carried out on the sludge generated in the Sotaquirá WWTP suggests that this sludge has potential agricultural use due to its high content of total organic matter, organic elemnts (carbon, phosphorus, and nitrogen), and low levels of heavy metals. The two stabilization treatments used allowed to establish a viable alternative in the sanitization of sludge for agricultural applications with some restrictions.
In the case of dewatering using drying beds, a category B biosolid was obtained after five months of treatment without affecting the physicochemical characteristics of the sludge. In contrast, alkaline stabilization with CaO concentrations of 7 %, 9 %, and 13 % reduced sludge pathogen loads, successfully yielding biosolids of the A category. Notably, the two low concentrations of CaO did not reach pH levels of 12 units as recommended Los huevos de helminto se eliminaron con estabilización alcalina, y con el tratamiento de deshidratación se redujeron a uno o cero. Los fagos somáticos se eliminaron con estabilización alcalina, pero se redujeron solamente a 3.52 log UFC/g con el método de deshidratación. La deshidratación en lechos de secado produjo biosólidos que pueden utilizarse en restauración de suelos, mientras la estabilización alcalina produjo biosólidos que se pueden usar con propósitos agrícolas. La estabilización alcalina con 9 % y 13 % de óxido de calcio redujo ostensiblemente los contenidos de nitrógeno y fósforo del lodo, mientras que el calcio al 7 % afectó menos la concentración de fósforo. Estos resultados indican que la deshidratación en lechos de secado es un protocolo efectivo de estabilización de lodos, que puede ser implementado en plantas pequeñas de tratamiento de aguas residuales, como la de Sotaquirá, Colombia.

Jaqueline Arleth Galvis López
Professional in food chemistry with a master's degree in biological sciences from the pedagogical and technological university of Colombia. Associate professor at the University of Boyacá, experience in research in the areas of physicochemical and environmental analysis.

Nuri Andrea Merchán Castellanos
Professional in Biological Sciences with PhD in Biotechnology from the University of Sao Paulo and undergraduate in Industrial Microbiology from the Universidad Javeriana. Research experience as a principal researcher and co-researcher in the areas of environmental and food microbiology, biotechnology and bioprospecting, applying tools of molecular biology, bioinformatics and bioprocesses.

Elsa Helena Manjarres Hernández
Biologist with a Master's Degree in Biological Sciences, Ph.D. student in Biological and Environmental Sciences of the Pedagogical and Technological University of Colombia. Experience in research in molecular biology and plant genetics.