Microbiological Control in Decontamination of Sludge from Wastewater Treatment Plant
1
Faculty of Forestry, Department of Soil Science, University of Forestry, 10 Kliment Ohridski Blvd., 1797 Sofia, Bulgaria
2
Faculty of Ecology and Landscape Architecture, University of Forestry, 10 Kliment Ohridski Blvd., 1797 Sofia, Bulgaria
*
Author to whom correspondence should be addressed.
Processes 2022, 10(2), 406; https://doi.org/10.3390/pr10020406
Submission received: 15 December 2021
/
Revised: 11 February 2022
/
Accepted: 13 February 2022
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Published: 18 February 2022
(This article belongs to the Special Issue Sustainable Shift to Circular Solutions for Water and Waste Management)
Abstract
:Dynamics of the microbiological indicators of fresh sludge from wastewater treatment plants with a concentration of CaO, 10% and 20%, and ash, −30% and 50%, and treated with quicklime, ash, and microbial fertilizer for a 50-day period of composting were studied. The influence of temperature, water content, and oxygen on the development of microbes was established in laboratory conditions. Microbiological analysis included the determination of non-pathogenic (non-spore-forming bacteria, bacilli, actinomycetes, micromycetes, bacteria digesting mineral nitrogen), and pathogenic (Salmonella, Listeria, Escherichia coli, Enterococcus, Clostridium perfringens) microorganisms. Of the beneficial microflora in the sludge before treating, the amount of non-spore-forming bacteria was the highest, followed by bacilli and micromycetes. It was found that actinomycetes were absent in the untreated sludge. Clostridium perfringens occupied a major share in the composition of the pathogenic microflora, followed by Escherichia coli, Enterococcus, and Listeria. The best results for decontamination of the sludge were obtained by adding 20% quicklime and 50% ash. Alkalization of the sludge after treatment led to the destruction of pathogenic microflora but also reduced the number of beneficial microorganisms. The decrease in pH during the study period determined the redevelopment of pathogenic microflora. Combined variants with lime or ash and microbial fertilizer showed better results for the development of non-pathogenic microflora and the destruction of pathogens.
1. Introduction
The management of sludge from wastewater treatment plants (WWTPs) in accordance with environmental principles is an extremely topical issue for the sustainable development of settlements. Its decision is directly related not only to the construction of wastewater treatment plants but also to the use of sludge as an alternative energy source (energy resource) and/or soil improver (raw material resource) [1].
In their use as a soil improver, decontamination and deworming of sludge from wastewater treatment plants is an extremely important pretreatment process. Sludge from wastewater treatment plants deactivated with quicklime can be used for agriculture purposes [2,3,4]. Pathogenic microorganisms and parasitic worm eggs can be spread by sediment [5,6]. Isolated in the sludge are pathogenic microorganisms such as: Salmonella sp., Listeria sp., Escherichia coli, Campylobacter sp., Clostridium sp., Yersinia sp., and others [4,7,8,9,10]. These microorganisms have a strong ability to constantly adapt to changes in the survival environment [11] and can be relatively resistant (especially spore-forming species such as Clostridium perfringens) to commonly used sludge stabilization methods [12]. According to a study by Dermendzhieva [3], in sludge from Bulgarian treatment plants, the number of fecal (coliforms, Escherichia coli) and intestinal pathogenic (Enterobacteriaceae, Salmonella) microorganisms was above the permissible norms in the Ordinance on the procedure for utilization of sewage sludge through their use in agriculture [13], which makes them unusable for fertilization in agriculture without applying methods for their decontamination. Co-composting of sewage sludge with lime effectively reduces and even destroys pathogens in the sludge [14,15,16]. According to Wong and Fang [16], the addition of 0.63% lime before sludge composting slightly improved microbial activity from temperature rise and CO2 release and did not significantly inhibit bacterial population and β-glucosidase, alkaline phosphatase, and dehydrogenase activity after 100 days of composting. These authors estimated the use of lime at <1.0% (w/w) for co-composting with sewage sludge. On the one hand, this concentration inhibited the development of beneficial groups of microorganisms in sludge to a small degree, but according to other authors [17], the most promising for a short period of time (about 1 month) is the decontamination of sludge by adding a 20% fine and 30% coarser fraction of quicklime. According to them, treatment with quicklime is particularly suitable for this purpose, as lime is not environmentally hazardous and is, at the same time, effective.
The removal of pathogenic microorganisms depends on the pH of the sludge, the period of liming, and the water content of the sludge [18]. The main role of liming is to sterilize the sludge from pathogenic microorganisms and parasites. The study by Santos et al. 2021 [19] confirmed that lime mud (LM) and calcined lime mud (CLM) can be used as drying additives and sanitizing agents for sewage sludge. According to these authors, the addition of calcined lime mud using a ratio of between 0.05 and 0.15 g CLM/gwb led to the complete elimination of microbiological contamination in almost all cases. Except for fertilization, the modified sludge with fly ash and loess satisfies the criteria for construction materials [20]. According to Jagaba et al. 2019 [21], incinerated sewage sludge ash (ISSA) can be used as a soil-stabilizing agent. These authors concluded that 7% ISSA additive effectively enhances the strength of soft soils. Some authors [22] have investigated the optimization of conditions for the production of activated carbon (chemical activation ratio, contact time, and activation temperature on the surface area of activated carbon) using sewage sludge as a raw material. According to Gheethi et al. 2018 [4], further treatment using technologies such as solar disinfection, air drying, and lime treatment of sewage effluents and biosolids generated from secondary treatment is necessary to reduce the pathogenic bacteria before reusing for agricultural purposes. Therefore, the usage of sludge in agriculture, construction, activated carbon production, and other purposes implies their decontamination by pathogenic microorganisms. Stabilization of sewage sludge is achieved by raising the pH values to create an alkaline environment that leads to a reduction in nutrient content [23]. In the treatment of sewage sludge with lime, Bina et al. [24] found that when the pH increased to 11–12, Salmonella was completely inactivated in the treated sludge after 2 hours, while the removal of up to 99% of fecal coliforms was obtained for two pH ranges. Lime treatment and pasteurization of sludge (50-day sanitation) are very effective methods for disinfection and creating safe sludge in terms of fecal coliform content [25]. In addition to liming and the use of ash, fertilization with microbial fertilizers also supports the development of beneficial microorganisms in substrates [26,27]. For the decontamination of sludge, it would be useful to study the impact of microbial fertilizers containing microorganisms with antimicrobial properties against pathogenic microbes in the sludge, as well as the detection of microorganisms that can accumulate and degrade heavy metals. Similarly, plant species can be studied that, in addition to having antimicrobial properties and accumulating heavy metals, can also be used to deodorize sludge.
The aim of the study was to conduct microbiological control using various methods (treatment with lime, ash, microbial fertilizer, freezing, and thermal effect alone or in combination) to decontaminate sewage sludge. The literature review presented above confirms the effectiveness of liming as a method for the decontamination of sludge, but the proposed methods differ depending on the concentration and size of the quicklime fraction, the treatment period, and the degree of destruction of pathogenic microorganisms. The use of ash [20,21] and microbial fertilizer for the decontamination of sludge are less studied. In the present research, the analysis of microbiological indicators in dynamics for a period of 50 days after treatment with different ameliorants-alone, in combination, and under different temperature conditions-provides guidelines for creating an effective methodology based on the combined treatment (not only the independent use of ameliorants for a short period) of sewage sludge, compost, and organic waste. This process can reduce and completely destroy pathogenic microorganisms while monitoring the preservation of beneficial microorganisms in the treated substrates. Usually, the microbiological control of sewage sludge is monitored only for their decontamination for safe use; our study compared changes in the amount of non-pathogenic and pathogenic microflora after treatment, which is a prerequisite for the safe and effective use of sludge as a soil improver.
2. Materials and Methods
Sludge from municipal WWTP in Blagoevgrad was used. The wastewater treatment technology included mechanical and biological stages. The sludge underwent thickening, anaerobic stabilization in open digesters, dewatering in belt filter presses, and, finally, deposition to sludge drying beds. Sludge (initial indicators: pH = 6.23; water content 90%, t = 30 °C) obtained from the belt filter presses of the WWTP-Blagoevgrad was studied according to microbiological indicators, in dynamics, composted in greenhouse conditions (air temperature: t-day = 28–30 °C, t-night = 17–18 °C; air humidity: 50–60%), in plastic packages (containers) from 2 L, and treated with quicklime and ash in different concentrations. For some of the variants, fertilization with microbial fertilizer (Baikal EM-1) containing lactic acid and photosynthetic bacteria, nitrogen-fixing bacteria, and Saccharomyces was applied. Each package contained 100 g of dry matter, comprising: 10 g CaO (“Sludge + 10% CaO”), 20 g CaO (“Sludge + 20% CaO”), 30 g ash (“Sludge + 30% ash”), 50 g ash (“Sludge + 50% ash”), and 100 mL of diluted microbial fertilizer solution (for all variants with microbial fertilizer). In laboratory conditions, the sludge samples were placed in different temperatures and aerobic and anaerobic conditions for different periods of time before their analysis. The scheme of variants and the period of microbiological analysis are presented in Table 1a,b, and the composting of the variants in greenhouse conditions is illustrated in Figure 1.
Microbiological analyses were performed by the method of limiting dilutions, inoculating, and culturing the nutrient media [28]: non-pathogenic microflora: Nutrient Agar for non-spore-forming bacteria and bacilli, MRS Agar for lactobacilli, Actinomycetes Isolation Agar for actinomycetes and bacteria digesting mineral nitrogen, and Czapek Dox Agar for micromycetes; pathogenic microflora: Desoxycholate Citrate Agar for Salmonella sp., ChromoBio Listeria Agar for Listeria sp., Endo Agar for Escherichia coli and coliforms (oxidase confirmatory test), ChromoBio Enterococcus Agar for Enterococcus, and Perfringens Agar (TSC and Perfringens Selective Supplement) for Clostridium perfringens. A jar with a reagent to generate an anaerobic medium was used to isolate anaerobes. The results are presented as colony-forming units (CFU) recalculated per 1 g of substrate given the amount of inoculation and dilution used [29]. The microbiological analyses were performed in the microbiological laboratory of the University of Forestry, Sofia, Bulgaria, using chemical reagents supplied by Valerus Ltd., Sofia, Bulgaria, nutrient media for microbiological purposes supplied by Optim Co Ltd., Sofia, Bulgaria, and microbial fertilizer Baikal EM-1 supplied by Provision Ltd., Sofia, Bulgaria.
For observing the temperature of the sludge, a temperature probe (model Sv 218) was used for the determination of the water content, a moisture meter for the greenhouse (model Sv 218), and a moisture scale in the laboratory (model DBS 60-3). For the determination of pH (in water), a pH meter (model Portavo 902) was used.
3. Results and Discussion
Liming raised the pH values of the sludge to an alkaline environment: 12.65, “10% CaO”; 12.64, “10% CaO + microbial fertilizer”; 12.72, “20% CaO”; 12.74, “20% CaO + microbial fertilizer.” The addition of ash decreased the pH to a smaller degree: 6.97, “30% ash”; 7.01, “30% ash + microbial fertilizer”; 7.85, “50% ash”; 7.56, “50% ash + microbial fertilizer.” This trend was characteristic until the 10th day of the study, after which the pH values decreased to a neutral medium at the end of the experiment. The addition of microbial fertilizer lowered the pH of the sludge slightly (pH control = 6.23; pH sludge + microbial fertilizer = 6.12). The water content of the variants decreased from 90% at the beginning of the experiment to about 40% at the end of the experiment. The temperature of the variants increased by more than 10 °C until the end of the experiment (40–42 °C). These changes in physicochemical parameters affected the microbiocenosis of sludge.
Microbiological analysis of fresh sludge from the treatment plant showed the presence of specific and non-specific (pathogenic) microflora. The number of non-pathogenic groups of microorganisms (in greenhouse conditions) is presented in Table 2.
The amount of non-spore-forming bacteria and bacteria digesting mineral nitrogen was the highest, micromycetes and bacilli were less represented, and actinomycetes were absent in the studied sludge (except for the sample in which microbiological fertilizer was added). Liming increased the pH values of the sludge (alkaline medium to a pH of about 12), which led to a decrease in the number of non-pathogenic groups of microorganisms studied. This trend was characteristic until the 10th day of the study, after which the number of microbes slowly began to increase, with decreasing pH values over time. The addition of ash led to an increase in the biogenicity of the studied sludge (to a greater extent by the 10th day of the study), which was associated with increased activity of microorganisms that degrade carbon-containing organic compounds. A similar trend was found in the fertilization with microbial fertilizer, as the effect of the preparation increased with the extension of the study period. The trends depended on the added concentrations of quicklime and ash; in the samples with 20% CaO, the development of microorganisms was suppressed to a greater extent than in those with 10% CaO. The variant with 30% ash showed better results than the one with 50% ash, probably because the short-term addition of a high amount of carbon-containing ameliorant inhibited the growth of microorganisms to a greater extent. On the other hand, reducing the water content of the sludge increased the number of microorganisms.
Liming had a more limiting effect on the development of non-pathogenic groups of microorganisms compared with the addition of ash, reduction in water content (from 90% to 40%), and drying of sludge after increasing the temperature by about 10 °C at the end of the experiment. Combined variants with lime and microbial fertilizer, as well as ash with microbial fertilizer, showed better results for maintaining the development of non-pathogenic microflora compared with the use of lime or ash alone.
Pathogenic microorganisms were found in the studied variants: Escherichia coli and coliforms, Enterococcus, Clostridium perfringens, Listeria sp.; Salmonella sp. was absent (Table 3).
Before the start of the experiment, regarding the composition of pathogenic species in the sludge, the incidence of Clostridium perfringens (4000 cfu/g) was the highest, followed by Escherichia coli and coliforms (2400 cfu/g), Enterococcus (2100 cfu/g), and Listeria sp. (2000 cfu/g). It was found that Salmonella sp. was absent. Clostridium perfringens is a spore-forming species that makes it difficult to destroy. Liming led to a reduction in the number of pathogenic microbes (in all variants with liming) and their complete destruction by the 10th day of the experiment in the variants with 20% lime. Marinova et al. [17] also found the best decontamination of sewage sludge is with the addition of 20% and 30% quicklime. According to that study by these authors, Clostridium perfringens was destroyed in the indicated concentrations within 31 days after mixing the sludge with fine quicklime. The results of their research confirmed that in order to obtain an epidemiologically safe sludge for a month or less, it must undergo further treatment. The same authors found that the size of quicklime also has a different effect on the development of microorganisms-the larger the fraction is, the more difficult it is for it to homogenize with the sludge, so it is necessary to use a higher rate. According to Santos et al. 2021 [19], the addition of calcined lime mud (CLM) using a ratio of between 0.05 and 0.15 g CLM/gwb led to the complete elimination of microbiological contamination in almost all cases. Conversely, the use of lime mud (LM) did not seem to act effectively as a sanitary agent. Both LM and CLM showed a positive effect on the drying process compared with raw sludge, increasing the drying speed and reducing the drying time. Therefore, the creation of an alkaline environment is a method of purifying the sludge from pathogenic microorganisms. However, lowering the pH values (i.e., less alkaline environment, day 25; neutral environment, day 50) led to the redevelopment of pathogenic microorganisms, to a greater extent for Clostridium perfringens. The addition of 20% lime showed better results for the destruction of pathogenic microorganisms compared with the variants with 10% CaO. In terms of effectiveness, the 50% ash variants followed, which showed results that were almost twice for pathogen eradication compared with the addition of 30% ash. The combined variants—lime and microbial fertilizer and ash with microbial fertilizer—showed better results compared with the independent use of lime and ash. This trend provides grounds to use the combined variants with microbial fertilizer for two reasons: reducing the destruction of pathogens and activating the development of non-pathogenic microflora. Reducing the water content and drying the sludge had a significantly lower effect than liming, which is indicative of the fact that in the non-liming variants, the results remain close for the entire study period. Environmental conditions such as the active reaction of sludge, salt content, heavy metal content, and temperature can lead to changes in the course of microbial activity in the substrate and, consequently, affect the reproductive capacity of microorganisms [7]. The pathogen microorganisms studied have a strong ability to adapt continuously to changes in the survival environment [11] and may be relatively resistant (especially spore-forming species such as Clostridium perfringens) to commonly used sludge stabilization methods [19]. Other scientists have also found a positive effect on the decontamination of sewage sludge by applying lime [1,10,12,15,24]. On the one hand, for the complete destruction and prevention of the redevelopment of the pathogenic microflora, additional liming was needed after the 10th day to preserve the alkaline environment, but on the other hand, this reduced the number of beneficial microorganisms. Incinerated sewage sludge ash (ISSA) can be used as a soil-stabilizing agent; 7% ISSA additive can effectively enhance the strength of soft soils [21]. It is, therefore, necessary to combine liming with the addition of ash, microbial fertilizers, suitable plant species with bactericidal and deodorizing action, and other methods in the process of monitoring the development of microflora in sludge in dynamics. According to Gheethi et al. 2018 [4], further treatment using technologies such as solar disinfection, air drying, and lime treatment of sewage effluents and biosolids generated from secondary treatment is necessary to reduce the pathogenic bacteria before reusing for agricultural purposes.
The influence of temperature and oxygen on the development of pathogenic and non-pathogenic microflora was observed in laboratory conditions (Figure 2 and Figure 3).
At negative and higher positive (70 °C) temperatures, mesophilic microorganisms stopped their development and activity, and psychrophiles and thermophiles (which are less than mesophiles) actively developed in samples cultured at 28 °C. Thermal treatment (100 and 130 °C) without adjuvants reduced microbiological contamination in the sewage sludge [19]. By lowering the temperature to −20 °C and prolonging the cultivation time, the number of microbes decreased. An exception to this trend was found for Clostridium; their amount was lower at −20 °C for 6 h compared with the same temperature for 24 h, which is explained by their mechanisms for adaptation to environmental conditions. Anaerobic microorganisms were fewer than aerobic ones, with the exception of Clostridium perfringens, which are anaerobes, and lactobacilli, which are microaerophiles. Placing the samples at extreme temperatures and in an oxygen-free environment before analyzing reduces the development of pathogens but does not destroy them. The amount of Clostridium perfringens depends least on the influence of extreme temperature conditions, as they are spore-forming and can better adapt and survive changes.
Pictures of some isolated microorganisms in the sludge are presented in Figure 4.
4. Conclusions
The dynamics of non-pathogenic and pathogenic microorganisms were studied using sludge from the treatment plant after decontamination with quicklime and ash, in different concentrations and for a period of 50 days. Of the beneficial microflora in the sludge, the highest was the number of non-spore-forming bacteria and bacteria digesting mineral nitrogen, followed by bacilli and micromycetes; actinomycetes were absent. Clostridium perfringens had a major share in the composition of the pathogenic microflora, followed by Escherichia coli and coliforms, Enterococcus, and Listeria sp. It was found that Salmonella sp. was absent in the studied sludge.
The best results for the decontamination of sludge were given by the addition of 20% quicklime (10th day): complete destruction. At the same time, however, the creation of an alkaline environment led to a reduction in the number of beneficial microorganisms. On the one hand, additional liming was needed after the 10th day to completely destroy and prevent the regrowth of pathogenic microbes, and on the other hand, this probably led to a greater reduction in beneficial microorganisms. The decrease in water content and drying of the sludge during the study period showed a weaker effect on the dynamics of both pathogenic and non-pathogenic microorganisms compared with the effect of increasing pH.
The addition of ash reduced the pathogenic microflora but not their complete destruction. Better results in terms of pathogens destruction were obtained using 50% ash compared with 30%. In terms of non-pathogenic microflora, the 30% ash variant gave better results than the one with 50% ash; probably the short-term addition of a high amount of carbon-containing ameliorant initially inhibited the development of microorganisms to some extent. The combined variants showed better results for the development of non-pathogenic microflora and the destruction of pathogens compared with the use of only one ameliorant.
Cultivation at sub-zero and higher (70 °C) temperatures inactivated the development of mesophilic microorganisms, determined the formation of cultures of psychrophiles and thermophiles, which are less than mesophiles, and actively developed in samples cultured at 28 °C. By lowering the temperature to −20 °C and prolonging the cultivation time, the number of microbes decreased to a greater extent. An exception to this trend was found for Clostridium, which is spore-forming and survives better in extreme conditions.
The results show that the addition of ameliorants such as lime, ash, and microbial fertilizers, the creation of acidic aerobic and anaerobic environments, and the use of different cultivation temperatures are factors that can be used to reduce or destroy pathogenic microorganisms in sewage sludge, compost, and organic waste. Long-term research and a multifactorial approach are needed to identify suitable combined variants for both the complete decontamination of sludge and the preservation of beneficial microflora in order for sludge to be used safely as organic fertilizer in agriculture.
Author Contributions
The authors B.Z.M., P.G.P. and V.V.S. declare that they have participated equally in the preparation of the conceptualization, methodology, conduct of the research, writing and editing of the manuscript. All authors have read and agreed to the published version of the manuscript.
Funding
The research presented in the paper was carried out in the framework of the Clean & Circle Project BG05M2OP001-1.002-0019: “Clean technologies for sustainable environment–waters, waste, en-ergy for circular economy”, funded by the Operational Programme “Science and education for smart growth” 2014–2020, co-financed by the European union through the European structural and investment funds. We acknowledge the financial support of the project.
Institutional Review Board Statement
Not applicable.
Informed Consent Statement
Not applicable.
Data Availability Statement
The data presented in this study are available in the manuscript.
Acknowledgments
The research presented in the paper was carried out in the framework of the Clean & Circle Project BG05M2OP001-1.002-0019: “Clean technologies for sustainable environment–waters, waste, en-ergy for circular economy”, funded by the Operational Programme “Science and education for smart growth” 2014–2020, co-financed by the European union through the European structural and investment funds. We acknowledge the financial support of the project.
Conflicts of Interest
The authors declare no conflict of interest.
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Figure 1.
Composted variants in greenhouse conditions.
Figure 2.
Non-pathogenic microflora (cfu/g).
Figure 3.
Pathogenic microflora (cfu/g).
Figure 4.
Some isolated microorganisms in the composted variants.
Table 1.
Scheme of variants and period of microbiological analysis.
| Variants | Microbiological Analysis |
|---|---|
| 1a: In Greenhouse Conditions | Before starting the experiment; 10 h after starting the experiment; 10th day after starting the experiment; 25th day after starting the experiment; 50th day after starting the experiment |
| Sludge without quicklime (CaO) | |
| Sludge + 10% CaO | |
| Sludge + 20% CaO | |
| Sludge + microbial fertilizer + 10% CaO | |
| Sludge + microbial fertilizer + 20% CaO | |
| Sludge + 30% ash | |
| Sludge + 50% ash | |
| Sludge + microbial fertilizer + 30% ash | |
| Sludge + microbial fertilizer + 50% ash | |
| Sludge + microbial fertilizer | |
| 1b: In Laboratory Conditions | Microbiological Analysis |
| Sludge, +28 °C (aerobic cultivation) | Before starting the experiment, 24 h |
| Sludge, −4 °C (aerobic cultivation) | 24 h |
| Sludge, −20 °C (aerobic cultivation) | 3 h, 6 h, 24 h |
| Sludge, +70 °C (aerobic cultivation) | 6 h, 12 h |
| Sludge, +28 °C (anaerobic cultivation) | 7 days |
Table 2.
Quantity and qualitative composition of non-pathogenic microflora in raw sludge (cfu/g) in dynamics.
Table 2.
Quantity and qualitative composition of non-pathogenic microflora in raw sludge (cfu/g) in dynamics.
| Variants | Before Starting | Day 1, 10 h | 10th Day | 25th Day | 50th Day |
|---|---|---|---|---|---|
| In Greenhouse Conditions | Non-Spore-Forming Bacteria | ||||
| Sludge without quicklime (CaO) | 2200 | 2100 | 2200 | 2140 | 2120 |
| Sludge + 10% CaO | 1700 | 1520 | 1580 | 1640 | |
| Sludge + 20% CaO | 1480 | 1000 | 1120 | 1240 | |
| Sludge + microbial fertilizer + 10% CaO | 1800 | 1640 | 1700 | 1780 | |
| Sludge + microbial fertilizer + 20% CaO | 1550 | 1380 | 1460 | 1500 | |
| Sludge + 30% ash | 1800 | 1900 | 1880 | 1860 | |
| Sludge + 50% ash | 1600 | 1740 | 1700 | 1680 | |
| Sludge + microbial fertilizer + 30% ash | 1900 | 2040 | 2080 | 2080 | |
| Sludge + microbial fertilizer + 50% ash | 1850 | 2000 | 2080 | 2140 | |
| Sludge + microbial fertilizer | 2240 | 2300 | 2320 | 2340 | |
| In Greenhouse Conditions | Bacilli/Lactobacilli | ||||
| Sludge without quicklime (CaO) | 320 | 310 | 400 | 360 | 340 |
| Sludge + 10% CaO | 170 | 140 | 150 | 160 | |
| Sludge + 20% CaO | 110 | 80 | 100 | 100 | |
| Sludge + microbial fertilizer + 10% CaO | 200 | 160 | 180 | 180 | |
| Sludge + microbial fertilizer + 20% CaO | 170 | 120 | 140 | 160 | |
| Sludge + 30% ash | 190 | 260 | 240 | 220 | |
| Sludge + 50% ash | 140 | 200 | 180 | 180 | |
| Sludge + microbial fertilizer + 30% ash | 220 | 300 | 260 | 280 | |
| Sludge + microbial fertilizer + 50% ash | 210 | 280 | 260 | 260 | |
| Sludge + microbial fertilizer | 360 | 440 | 460 | 480 | |
| In Greenhouse Conditions | Actinomycetes | ||||
| Sludge without quicklime (CaO) | 0 | 0 | 0 | 0 | 0 |
| Sludge + 10% CaO | 0 | 0 | 0 | 0 | |
| Sludge + 20% CaO | 0 | 0 | 0 | 0 | |
| Sludge + microbial fertilizer + 10% CaO | 0 | 0 | 0 | 0 | |
| Sludge + microbial fertilizer + 20% CaO | 0 | 0 | 0 | 0 | |
| Sludge + 30% ash | 0 | 0 | 0 | 0 | |
| Sludge + 50% ash | 0 | 0 | 0 | 0 | |
| Sludge + microbial fertilizer + 30% ash | 0 | 0 | 0 | 0 | |
| Sludge + microbial fertilizer + 50% ash | 0 | 0 | 0 | 0 | |
| Sludge + microbial fertilizer | 20 | 20 | 20 | 20 | |
| In Greenhouse Conditions | Micromycetes | ||||
| Sludge without quicklime (CaO) | 700 | 600 | 700 | 680 | 660 |
| Sludge + 10% CaO | 300 | 220 | 240 | 260 | |
| Sludge + 20% CaO | 100 | 40 | 60 | 60 | |
| Sludge + microbial fertilizer + 10% CaO | 400 | 340 | 360 | 380 | |
| Sludge + microbial fertilizer + 20% CaO | 170 | 100 | 140 | 160 | |
| Sludge + 30% ash | 400 | 480 | 440 | 440 | |
| Sludge + 50% ash | 300 | 360 | 340 | 320 | |
| Sludge + microbial fertilizer + 30% ash | 500 | 600 | 580 | 560 | |
| Sludge + microbial fertilizer + 50% ash | 440 | 500 | 480 | 480 | |
| Sludge + microbial fertilizer | 700 | 800 | 760 | 780 | |
| In Greenhouse Conditions | Bacteria, Digesting mineral nitrogen | ||||
| Sludge without quicklime (CaO) | 1980 | 1920 | 2140 | 2120 | 2080 |
| Sludge + 10% CaO | 1780 | 1620 | 1680 | 1720 | |
| Sludge + 20% CaO | 1500 | 1320 | 1360 | 1360 | |
| Sludge + microbial fertilizer + 10% CaO | 1850 | 1740 | 1780 | 1800 | |
| Sludge + microbial fertilizer + 20% CaO | 1580 | 1440 | 1480 | 1480 | |
| Sludge + 30% ash | 1820 | 1920 | 1900 | 1880 | |
| Sludge + 50% ash | 1700 | 1800 | 1780 | 1780 | |
| Sludge + microbial fertilizer + 30% ash | 1900 | 2000 | 1960 | 1960 | |
| Sludge + microbial fertilizer + 50% ash | 1820 | 1900 | 1880 | 1860 | |
| Sludge + microbial fertilizer | 2060 | 2140 | 2120 | 2100 | |
Table 3.
Quantity and qualitative composition of pathogenic microflora in raw sludge (cfu/g) in dynamics.
Table 3.
Quantity and qualitative composition of pathogenic microflora in raw sludge (cfu/g) in dynamics.
| Variants | Before Starting | Day 1, 10 h | 10th Day | 25th Day | 50th Day |
|---|---|---|---|---|---|
| In Greenhouse Conditions | Escherichia coli and Coliforms | ||||
| Sludge without quicklime (CaO) | 2400 | 2400 | 2380 | 2320 | 2300 |
| Sludge + 10% CaO | 1200 | 620 | 800 | 1020 | |
| Sludge + 20% CaO | 0 | 0 | 60 | 160 | |
| Sludge + microbial fertilizer + 10% CaO | 1000 | 540 | 600 | 900 | |
| Sludge + microbial fertilizer + 20% CaO | 0 | 0 | 20 | 40 | |
| Sludge + 30% ash | 1500 | 1040 | 1220 | 1380 | |
| Sludge + 50% ash | 800 | 600 | 660 | 680 | |
| Sludge + microbial fertilizer + 30% ash | 820 | 700 | 740 | 780 | |
| Sludge + microbial fertilizer + 50% ash | 500 | 320 | 380 | 400 | |
| Sludge + microbial fertilizer | 1050 | 920 | 900 | 840 | |
| In Greenhouse Conditions | Enterococcus sp. | ||||
| Sludge without quicklime (CaO) | 2100 | 2100 | 2000 | 1960 | 1920 |
| Sludge + 10% CaO | 1000 | 540 | 680 | 800 | |
| Sludge + 20% CaO | 0 | 0 | 80 | 180 | |
| Sludge + microbial fertilizer + 10% CaO | 880 | 480 | 500 | 560 | |
| Sludge + microbial fertilizer + 20% CaO | 0 | 0 | 40 | 40 | |
| Sludge + 30% ash | 1200 | 960 | 980 | 1000 | |
| Sludge + 50% ash | 600 | 440 | 480 | 500 | |
| Sludge + microbial fertilizer + 30% ash | 700 | 540 | 560 | 580 | |
| Sludge + microbial fertilizer + 50% ash | 340 | 200 | 240 | 280 | |
| Sludge + microbial fertilizer | 800 | 680 | 660 | 660 | |
| In Greenhouse Conditions | Salmonella sp. | ||||
| Sludge without quicklime (CaO) | 0 | 0 | 0 | 0 | 0 |
| Sludge + 10% CaO | 0 | 0 | 0 | 0 | |
| Sludge + 20% CaO | 0 | 0 | 0 | 0 | |
| Sludge + microbial fertilizer + 10% CaO | 0 | 0 | 0 | 0 | |
| Sludge + microbial fertilizer + 20% CaO | 0 | 0 | 0 | 0 | |
| Sludge + 30% ash | 0 | 0 | 0 | 0 | |
| Sludge + 50% ash | 0 | 0 | 0 | 0 | |
| Sludge + microbial fertilizer + 30% ash | 0 | 0 | 0 | 0 | |
| Sludge + microbial fertilizer + 50% ash | 0 | 0 | 0 | 0 | |
| Sludge + microbial fertilizer | 0 | 0 | 0 | 0 | |
| In Greenhouse Conditions | Listeria sp. | ||||
| Sludge without quicklime (CaO) | 2000 | 2000 | 1960 | 1940 | 1920 |
| Sludge + 10% CaO | 0 | 0 | 40 | 40 | |
| Sludge + 20% CaO | 0 | 0 | 0 | 40 | |
| Sludge + microbial fertilizer + 10% CaO | 0 | 0 | 20 | 20 | |
| Sludge + microbial fertilizer + 20% CaO | 0 | 0 | 0 | 20 | |
| Sludge + 30% ash | 0 | 0 | 40 | 60 | |
| Sludge + 50% ash | 0 | 0 | 20 | 40 | |
| Sludge + microbial fertilizer + 30% ash | 0 | 0 | 20 | 40 | |
| Sludge + microbial fertilizer + 50% ash | 0 | 0 | 20 | 20 | |
| Sludge + microbial fertilizer | 900 | 800 | 700 | 620 | |
| In Greenhouse Conditions | Clostridium perfringens | ||||
| Sludge without quicklime (CaO) | 4000 | 4000 | 4020 | 4060 | 4080 |
| Sludge + 10% CaO | 2000 | 1620 | 1700 | 1720 | |
| Sludge + 20% CaO | 0 | 0 | 80 | 120 | |
| Sludge + microbial fertilizer + 10% CaO | 400 | 300 | 320 | 360 | |
| Sludge + microbial fertilizer + 20% CaO | 0 | 0 | 60 | 100 | |
| Sludge + 30% ash | 3000 | 2720 | 2760 | 2800 | |
| Sludge + 50% ash | 1800 | 1600 | 1640 | 1680 | |
| Sludge + microbial fertilizer + 30% ash | 2600 | 1800 | 1900 | 2100 | |
| Sludge + microbial fertilizer + 50% ash | 200 | 120 | 140 | 160 | |
| Sludge + microbial fertilizer | 4000 | 4000 | 4000 | 4000 | |
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Malcheva, B.Z.; Petrov, P.G.; Stefanova, V.V. Microbiological Control in Decontamination of Sludge from Wastewater Treatment Plant. Processes 2022, 10, 406. https://doi.org/10.3390/pr10020406
AMA Style
Malcheva BZ, Petrov PG, Stefanova VV. Microbiological Control in Decontamination of Sludge from Wastewater Treatment Plant. Processes. 2022; 10(2):406. https://doi.org/10.3390/pr10020406
Chicago/Turabian StyleMalcheva, Boyka Z., Petar G. Petrov, and Veneta V. Stefanova. 2022. "Microbiological Control in Decontamination of Sludge from Wastewater Treatment Plant" Processes 10, no. 2: 406. https://doi.org/10.3390/pr10020406
APA StyleMalcheva, B. Z., Petrov, P. G., & Stefanova, V. V. (2022). Microbiological Control in Decontamination of Sludge from Wastewater Treatment Plant. Processes, 10(2), 406. https://doi.org/10.3390/pr10020406
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