The evaluation of a sanitary WAS is based on the inference of the presence of pathogenic bacteria and eggs of intestinal parasites. Its purpose is to detect the so-called sanitary indicator. National and international regulations (European Commission, 2000, Council Directive 86/278/EEC, US EPA, 2003) dictate that sludge should be stabilized and hygienized before land introduction.
Sewage sludge is characterized by a large number and diversity of microorganisms, including pathogens. The recommended indicators for sanitary assessment of sewage sludge are Salmonella sp., Escherichia coli, faecal coliforms, enteric viruses, sulfate-reducing Clostridia, and parasite eggs. Therefore, it is necessary to look for methods enabling their efficient reduction or total elimination. As a consequence, these techniques allow the possible use of sludge in agriculture. Therefore, the knowledge of selected hygienic indicators in sewage sludge permits making decisions about its use.
The removal of pathogenic bacteria and helminths from waste activated sludge by hydrodynamic destruction and freezing/thawing can help considerably in the reduction of the transfer operation of illnesses. The mechanical process and freezing/thawing can cause the destruction of microorganisms and thus contribute to a partial hygienization of WAS, which was confirmed by the results of the microbiological and parasitological analyses (Figs. 2 and 3).
After hygienization by hydrodynamic cavitation and dry ice, a gradual decrease in the population count of bacteria was observed (Fig. 2). According to Polish law, Salmonella sp. removal is imperative for the agricultural use of sewage sludge (Minister of the Environment, 2015). After 30 minutes of hydrodynamic cavitation of sludge, microorganism disintegration resulted in the elimination of Salmonella sp. from 4.47 log to 4.24 log cfu/gd.w. The number of Salmonella sp. in 1 gd.w. (the volume ratio of WAS to dry ice was 1:1; freezing/thawing) was reduced by approximately 14.5% (Fig. 3).
Depending on disintegration by hydrodynamic destruction and freezing/thawing, a reduction number of E. coli in WAS of approximately 42.3% (from 5.88 log to 5.64 log cfu/gd.w.) and 13.7% (5.88 log to 5.82 log cfu/gd.w.) - was observed, respectively (Figs. 2 and 3).
A higher removal efficiency of Escherichia coli during low-frequency ultrasound disintegration was obtained by Hawrylik (2019). She concluded that ultrasound has an effective impact on E. coli, in which a nearly 70% decrease in the amount of bacteria was obtained after 20 minutes of ultrasound at 40 kHz. Using acidification as a pretreatment method, Fukushi et al. (2003) found an effective reduction of Salmonella spp. in sewage sludge. Methods using chemical oxidants (advanced oxidation processes) have also positive effects in the elimination of microorganisms. After 2h of reaction of persulfate (0.5 mM) activated by solar energy, a unit reduction of 6.0 log of bacteria in sewage sludge was registered (Ferreira et al. 2020). Luukkonen et al. (2020) used peracetic acid for the conditioning of municipal wastewater sludge. They concluded that with the increase in the dose of peracetic acid to 480 mg/l, a decrease in Salmonella sp. to an amount acceptable for use of such sludge as a fertilizer in Finland occurred. Moreover, using alkalization and ultrasound pretreatment methods on sludge, disintegration deactivations of E. coli and sulfite-reducing Clostridia were obtained (Martín-Díaz et al. 2017). Studies on the amount of energy input with microwaves (up to 7 kWh) into the sludge showed an approximately 5.90 log removal value of the analysed pathogens (Mawioo et al. 2017).
For an organism to be considered an indicator, it should be characterized by common presence and survival in raw sludge. In addition, it must be easily cultured and easily identified. Therefore, it is difficult to choose one species of bacteria, which is the reason for using several indicator organisms. A potential indicator of the sanitary quality of sewage sludge, as mentioned in the literature (Sidhu and Toze 2009), is Clostridium perfringens. On the basis of microbiological tests, an elimination of Clostridium perfringens bacteria by each method of disintegration was observed. The number of Clostridium perfringens rods in WAS was reduced by 19.6% (after 30 minutes of disintegration by hydrodynamic cavitation), 7.8% (dry ice) and 38.4% (hybrid disintegration) (Figs. 2 and 3).
Similar effects were obtained for a decrease in the overall number of coliphages (Martín-Díaz et al. 2016). After 30 minutes of cavitation processes, the number was reduced by approximately 19.3%, while dry ice had a destructive effect of approximately 14.9% (Figs. 2 and 3). After the hybrid process, the number of coliphages changed from 5.77 log to 5.49 log cfu/gd.w. (Figs. 2 and 3).
The indicators of Ascaris sp., Trichuris sp., and Toxocara sp. are also being used in sanitary assessments of sewage sludge. Such indicators allow us to assess the agricultural usefulness of sewage sludge. Unfortunately, there is still a lack of sufficient information on their content in wastewater and sewage sludge and their survival during wastewater treatment processes.
Hydrodynamic disintegration and dry ice destroyed the eggs of helminths. The disintegration of WAS by hydrodynamic cavitation resulted in 26.4%, 22.8% and 29.3% reductions in Ascaris sp., Trichuris sp., and Toxocara sp., respectively (Figs. 2 and 3). As a result of disintegration by dry ice, the overall egg number decreased. A decrease from 2.08 log to 2.03 log eggs/kgd.w was observed for Ascaris sp., a decrease from 1.96 log to 1.88 log eggs/kgd.w. was observed for Trichuris sp. and a decrease from 3.05 log to 2.34 log eggs/kgd.w. was observed for Toxocara sp. (Figs. 2 and 3). A higher elimination effect was observed after the hybrid process was used.
In connection with the above, it can be clearly stated that the studied methods of disintegration contribute to the partial hygienization of sewage sludge. This is an important aspect because in activated sludge technology, the next stage (in most wastewater treatment plants - WWTPs) is the process of anaerobic stabilization.
The most commonly used process in WWTPs is stabilization under mesophilic conditions, which contributes to the decomposition and degradation of organic matter, biogas production and the diminution of pathogens. This process, unlike thermophilic digestion, has a low efficiency in the scope of sludge hygienization (Ruiz-Hernando et al. 2014; Carrere et al. 2016; Martín-Díaz et al. 2020). A low effectiveness of pathogenic bacterial inactivation in sewage sludge produced in Swedish treatment plants after digestion procedures (both thermophilic/mesophilic processes) was observed by Sahlström et al. (2004). Therefore, it is justified to look for solutions that contribute to the elimination of bacteria and parasite eggs before the fermentation process. One such possibility may be the use of disintegration methods. As we have shown in our earlier works (Grübel and Suschka 2015; Machnicka et al. 2015; Suschka and Grübel 2016; Grübel and Machnicka 2020; Li et al. 2021), the disintegration process effectively releases organic matter into the supernatant liquid of WAS, which contributes to the increased production of biogas. In connection with the above, we have undertaken research to determine the effectiveness of disintegration in removing selected bacterial indicators and parasite eggs.
According to the methodology (Fig. 1), the anaerobic stabilization process was performed on WAS with the addition of sludge after the hybrid disintegration process.
Analysing the above graph (Fig. 4), it can be noticed that with the increase in the volume of disintegrated WAS in the fermentation mixture, the reduction of the analysed indicators increases. The reduction efficiency of Salmonella sp., Escherichia coli, Clostridium perfringens and coliphages was better (from 1–23%) in mixtures with WASD compared to the control sample (70% WAS + 30% DS). Obviously, introducing only disintegrated sludge into the digestion chamber (70% WASD + 30% DS) is not economically and energetically justified from the technological point of view. Our previous research shows (Grübel and Suschka 2015) that the optimal dose of WASD is 30–40%, which definitely affects the production of biogas and biogas yield and is still economically justified.
Therefore, taking into account only the limited use of WASD in the fermentation mixture (30% vol.), the hygienisation efficiency in relation to bacterial indicators was Salmonella sp. – 11%, Escherichia coli – 13%, Clostridium perfringens – 2% and coliphages – 12%.
Similar effects were achieved with parasite eggs. Taking into account only 30% of the volume of WASD in the fermentation mixture, a reduction of 10% for Ascaris sp. and Toxocara sp. and 9.5% for Trichuris sp. in comparison to the blank sample (70% WAS + 30% DS) was attained.
Our investigations demonstrated an increase in the elimination of coliphages from 2.8–21.6% after the introduction of WASDH to fermentation processes. This result corresponds well to the results after mesophilic fermentation observed by Mandilara et al. (2006), which showed a reduction of 8.5%. Mocé-Llivina et al. (2003) claims that phages were much more resistant to thermal inactivation than bacterial indicators, with the exception of sulfite-reducing Clostridia spores. Somatic coliphages were significantly more resistant than Salmonella choleraesuis, E. coli, F-specific RNA phages and enteroviruses. The coliphages survived markedly better than Salmonella choleraesuis, and the scope of inactivation indicated that naturally occurring bacteriophages can be used to monitor the deactivation of E. coli and Salmonella sp. Pino-Jelcic et al. (2006) observed the log inactivation of the faecal coliform of approximately 4.2 ± 0.4 for microwaved/digested sludge, whereas for conventionally heated/digested sludge and the control, the reductions were only 2.9 ± 0.5 and 1.5 ± 0.5, respectively. Less log deactivation was noted for Salmonella spp. in microwaved/digested sludge - approximately 2.0 ± 0.3, whereas for conventionally heated/digested sludge and the control, the reductions were 1.9 ± 0.2 and 1.1 ± 0.3, respectively. Similarly, Hong et al. (2006) concluded from bench-scale fermentation that the introduction of microwaved disintegrated sludge to an anaerobic digester was more efficient in the inactivation of faecal coliforms (average of ≥ 2.66 log reduction) in comparison to chambers fed with untreated sludge and externally heated sludge. They observed that Class A sludge can be produced only when the sludge before stabilization is heated using microwaves to 65°C.
The research by Cella et al. (2016) showed an increase in faecal coliform removal from wastewater sludge depending on microwave intensification. In the fermenter chamber with the addition of microwaved sludge, 73.4% removal of coliforms was observed. Pike et al., 1988 described successful inactivation of Salmonella duesseldorf by mesophilic and thermophilic digestion but incomplete destruction of Ascaris suum ova. They concluded that the retention period influences the effectiveness of inactivation, and an increase in temperature from 36°C to 48°C increases the effectiveness by 3–4 times. Similar conclusions were obtained by Scheinemann et al. (2015). They found that 56 days of fermentation at 37°C were required for a complete reduction of Ascaris suum ova. Moreover, Maya et al. (2010) noted that temperature plays an essential role in the survival of Ascaris eggs. Mesophilic processes are inefficient at totally eliminating viable nematode eggs (Venglovsky et al. 2006). Similarly, Wagner et al. (2008) and Orzi et al. (2015), found that the temperature and physico-chemical properties of the sludge in the fermenter influence the inhibition of microorganisms.
Very often, after pretreatment and anaerobic processes, there is a regrowth of bacterial indicators. According to Erkan and Sanin (2013) and Leppeuple (2004), factors influencing the regrowth of bacteria are deficiency of nutrients, oxidation and osmotic stress and the presence of toxic substances. They are associated with the entry into the viable state of microorganisms, but they are not suitable for cultivation. This effect has not been found in bacteriophages. Martín-Díaz et al. (2017) concluded that alkali pretreatment and alkali and ultrasound pretreatments followed by anaerobic digestion repeatedly increased the number of Clostridia.
According to the methodology, the impact of the hybrid method of disintegration on the mesophilic fermentation process (biogas production and methane concentration) was determined (Table 1).
Table 1. Impact of mixture composition on cumulative biogas production after mesophilic process.
WAS – waste activated sludge; WASDH – sludge after hybrid disintegration; DS – digested sludge (inoculum); ±standard deviation
The obtained results confirmed that hybrid disintegration pretreatment (hydrodynamic cavitation and dry ice freezing/thawing) of only a part of WAS was substantiated. The volume of WASDH investigated was 10–70% in the digestion mixture. The highest effects of cumulative biogas production were obtained for 30% and 40% WASDH with increases of approximately 61.1% and 62.2%, respectively (Table 1).
The increasing amount of sewage sludge and the legislative regulation of its disposal have caused the need for developing new technologies to effectively process sewage sludge hygienization.