Table of Contents
Sewage effluent is defined as treated or untreated wastewater generated from treatment plant. The treated sewage is classified based on its origin in domestic sewage, hospital sewage and industrial wastewaters. Domestic sewage is a complex mixture containing water together with organic and inorganic constituents and large numbers of pathogenic bacteria as well as viruses and parasites. Hospital sewage is that coming from the hospitals and medical centres and includes sewage and wastewater resulting from the cleaning of laboratories and other facilities. Antibiotics, disinfectants and antibiotic-resistant bacteria are the major constituents in these wastes (due to their major use in hospital practice).
Industrial wastewaters are unwanted wastewater from the industrial operation such as chemical, electrochemical, electronic, petro-chemical and food-processing industries (US EPA 2009). These wastewaters are associated with high concentrations of dissolved metal salts (heavy metals). The sewage sludge is the solid, semisolid or liquid residue generated during the sewage treatment processes. The term sewage sludge has been replaced recently by the term bio solids. Bio solids represent sewage sludge that has been treated by advanced processes which included aerobic and anaerobic, heat or lime treatment and has met standards required for beneficial use.
Treated sewage and biosolids contain high concentrations of nutrients, which improve plant growth and soil properties. However, it has pathogenic microorganisms such as bacteria, protozoa, viruses and parasites that can cause several diseases. Land application of treated sewage and biosolids creates a potential for human exposure to these organisms through direct and indirect contact. Therefore, to protect public health from these organisms, many countries have regulated the use and disposal of treated sewage and bio solids.
Most Common Bacteria in Sewage-Treated effluent and Biosolids
Treated sewage and biosolids contain many pathogenic microorganisms, the most important are those transmitted by the faecal–oral route, which includes bacteria, viruses and parasites.
There is a wide spectrum of pathogenic bacteria that has been detected in the treated sewage and biosolids, many of which are enteric in nature.
- V. cholera,
- Leptospira spp.,
- Salmonella spp.,
- C. jejuni, E. coli O157:H7,
- Y. enterocolitica
- Shigella sp.
- B. cereus,
- Enterobacter spp.,
- Klebsiella spp.,
- C. perfringens,
- L. monocytogenes,
- P. aeruginosa,
- S. aureus and
- Streptococcus spp.
are the minor concerns which are considered opportunistic pathogens that cause disease only in debilitated or immunologically compromised individuals.
Disinfection Processes of Sewage‑Treated Effluents
The most common disinfection processes of treated sewage are discussed below.
One of the most effective disinfectants used in water disinfection is ozone. This is because ozone has high ability to destroy pathogenic cells through an irreversible physiochemical action. Ozonation destroys the cell wall of the bacteria as well as semi-permeable membrane. The destruction in the cell wall and membrane leads to the bacterial cell death (Facile et al. 2000). Tripathi et al. (2011) claimed that 5 min of exposure at a concentration of 10 mg ozone L−1 was suitable for the inactivation of pathogenic bacteria by 95–98%. Previous studies reported that ozone effectively removes TC and FC from sewage-treated effluents. Battaler et al. (2005) found that the ozone disinfection of secondary effluents at concentration 4.7 mg L−1 for 5 min had eliminated TC and FC (Faecal coliforms). At 21.4 mg L−1 the bacteria that resisted for chlorine such as P. aeruginosa decreased by 2 log reduction after 5 min of disinfection process by ozone.
Disinfection of treated sewage by ozone is applied because the use of ozone is cheap and low energy is needed. Nonetheless, the effectiveness of disinfection using ozone depends on the ozone dose, the ozone demand, the quality of the effluent and the transfer efficiency of the ozone system (Paraskeva and Graham 2002). The COD and total suspended solids (TSS) of treated sewage might affect the efficiency of disinfection process by ozone (Janex et al. 2000).
The properties of the treated sewage might induce the microbial resistance for the ozone as noted for Enterococcus sp., Clostridium sp. and Salmonella spp. which exhibit resistance to ozonation (Xu et al. 2002).
Solar disinfection (SODIS)
SODIS-based technologies are an efficient approach for the reduction of pathogenic microorganism in the water due to high availability of solar radiation and sustainable nature of these water treatment methods (Gomez-Couso et al. 2009). According to WHO (2002), SODIS depends on using transparent polyethylene terephthalate (PET) bottles and then exposing to the sun for a period between 4 and 8 h. Meierhofer and Wegelin (2002) recommended that PET bottles containing untreated raw water should be exposed to direct sunlight for at least 6 h. Bacteria, viruses, Giardia and Cryptosporidium cysts, and parasite eggs could be inactivated through the combination of ultraviolet radiation and elevated water temperature. Large field tests of SODIS were conducted in a number of countries in South America, Africa, and Asia in the 90s (Acra 1990).
The destruction of the bacterial cells takes place due to the combination of UV radiation and high temperature which has high potential to destroy the cell membrane (Al- Gheethi et al. 2015). However, SODIS has no efficiency for the reduction of chemical pollutants in the water. The sunlight has been reported as the single most important disinfection factor in the stabilization pond (Leduc and Gehr 1990; Maynard et al. 1999). Three main mechanisms are involved during the SODIS simultaneously, included the absorption of solar UV-B by microorganism DNA which causes direct damage by pyrimidine dimer formation. The process is independent of oxygen and other pond conditions.
The second mechanism depends on the absorption of UV-B and some shorter wavelength UV-A by cell constituents including DNA (called endogenous photo-sensitizers). The activated constituents react with oxygen to form highly reactive photo-oxidising species that damage genetic material within the cell or viral particle. The third mechanism involves absorption of a wide range of UV and visible wavelengths in sunlight by extra-cellular constituents of the pond medium (exogenous photo-sensitisers—notably humic material) (Jagger 1985).
Reduction of Pathogenic Bacteria in Biosolids
Sewage treatment processes can be classified as primary and secondary processes. In primary treatment, solids are mainly removed mechanically from untreated sewage. Secondary treatment is a biological process in which decomposers are utilized to remove biodegradable pollutants. Decomposers are organisms such bacteria and fungi that get energy and nutrients by digesting waste matter in the sewage. In the activated sludge process, sewage is pumped into a large tank where aerobic microorganisms decompose the organic matter (WHO 2002). Chemical treatment is sometimes used and it encourages small particles and dissolved substances to form large particles which facilitate separation. This is called chemical precipitation. Sludge is formed when these larger particles clump together during suitable separation methods.
Here we will discuss two methods which are used to remove pathogens from Biosolids.
Anaerobic digestion (AD) is a process by which microorganisms break down organic matter, producing various gases and a reduced volume of semi-solid residue. The gases produced, called ‘biogas’ or ‘digester gas,’ include a high percentage of methane, which can be burned to produce heat and/or electricity. Biogas from anaerobic digestion of biosolids and other organic residuals is a renewable, green fuel. The semi-solid residue from the digestion process, called ‘digestate’ (and/or ‘biosolids,’ if it is derived from wastewater solids and meets regulatory standards for use on land) is a stabilized material that can be used as a soil amendment, for animal bedding, or other uses, depending on the levels of further treatment.
Anaerobic digestion is a biological process that uses bacteria that function in an oxygen-free environment to convert volatile solids into carbon dioxide, methane and ammonia.
Those reactions take place in an enclosed tank that may or may not be heated, because the biological activity consumes most of the volatile solids needed for further bacterial growth (US EPA 2003). The mesophilic anaerobic digestion (MAD) of biosolids has been reported to produce biosolids Class B, this treatment process is common in USA.
Telles et al. (2002) investigated the reduction of TC, FC, P. aeruginosa and FS by anaerobic digestion of sewage sludge generated from STP in Maringá-Paraná, Brazil. The study showed that 99.9% of FS, 96.3% of TC and 95% of P. aeruginosa were reduced at the end of the treatment process. These findings revealed the potential of anaerobic digestion in the reduction of pathogenic bacteria from the biosolids. Wakelin et al. (2003) showed that FC was reduced from 7.5 log10 CFU g−1 in the raw sewage to 6.3 log10 CFU g−1 in dewatered biosolids after mesophilic anaerobic digestion process.
The main pathogen-reducing factor during thermophilic anaerobic digestion is temperature in relation to time, while the competition among microorganisms for nutrients is the limiting factor that reduces pathogen amounts in anaerobic mesophilic treatment of biosolids (Smith et al. 2005). It can be noted that the thermophilic treatment is more efficient than the mesophilic because the biosolids have high contents of nutrients; therefore, the competition between microorganisms in the biosolids is weak. Carrington (1998) also elucidated that temperature is not the main factor in mesophilic anaerobic digestion processes at 35 °C, but this process produces fatty acids and other products that are lethal to many pathogenic bacteria.
USEPA ultimately recommended this process, shown in the figure, as equivalent to a PFRP (Processes to Further Reduce Pathogens) one when “Sewage sludge is treated in the absence of air in an acidogenic thermophilic reactor and a mesophilic methanogenic reactor connected in series. The mean cell residence time shall be at least 2.1 days (± 0.05 d) in the acidogenic thermophilic reactor followed by 10.5 days (± 0.3 d) in the mesophilic methanogenic reactor. Feeding of each digester shall be intermittent and occurring no more than 4 times per day (no less than every 6 hours). The mesophilic methanogenic reactor shall be fed in priority from the acidogenic thermophilic reactor. Between two consecutive feedings temperature inside the acidogenic thermophilic reactor should be between 49oC and 60oC with 55oC maintained during at least 3 hours. Temperature inside the mesophilic methanogenic reactor shall be constant and at least 37oC (U.S.EPA, 2003).”
Lyonnaise des Eaux (ID’s ultimate parent company – name is now Suez) worked with EPA’s PEC for over four years. They assembled a test plan, constructed a pilot plant, and selected experimental and analytical procedures in consultation with the PEC. Finally the PEC observed some of the testing. The pilot plant testing of their process was conducted at Indianapolis’ Belmont Wastewater Treatment Plant using a blend of 60 percent (by vol.) thickened primary sludge and 40 percent (by vol.) thickened waste activated sludge. A 1 m3 (260 gal) tank was employed for the thermophilic unit and a 3 m3 (800 gal) tank for the mesophilic one.
The data showed an interesting combination of bactericidal effects. The acidogenic-thermophilic digester operated at temperatures between 48.6oC and 55oC with 55oC maintained for at least 21.6 hours during each detention time of 2.1 days. The combination of a short detention time with thermophilic temperatures enhanced the growth of acidogenic bacteria resulting in high levels of volatile fatty acids, free ammonia and low pH. Additional pathogen removal occurred in the methanogenic-mesophilic digester with the longer detention time and contact with ammonia, residual volatile acids and other chemicals.
Lynonnaise noted that other authors reported significant inactivation of heat resistant pathogens during methanogenic-mesophilic digestion after weakening during the previous acidogenicthermophilic digestion stage. It appears that the hydraulic and feeding mode of the digesters impacted the efficiency of pathogen removal. Semi continuous flow reactors with a ‘draw and fill’ feeding protocol virtually eliminated the potential for pathogens short-circuiting. Mixing is also critical in achieving optimum pathogen destruction.
Proper mixing efficiently dissipates heat within the digester preventing temperature gradients, dead spaces and hot spots. The relatively high level of volatile solids in the raw sludge is likely responsible for the higher levels of volatile fatty acids and free ammonia in both reactors. These chemicals together with the high temperature in the acidogenic-thermophilic digester contributed to the large reduction of pathogens.
In aerobic digestion, biosolids are biochemically oxidized by bacteria in an open or enclosed vessel. Under proper operating conditions, the volatile solids in biosolids are converted to CO2 and H2O (US EPA 2003). The PSRPs (Processes to Significantly Reduce Pathogens) described aerobic digestion as follows: Sewage sludge is agitated with oxygen to maintain aerobic condition for indigenous cell. Time and temperature shall be between 40 days at 20 °C and 60 days at 15 °C. Aerobic digestion carried out according to the part 503 requirement typically reduces pathogenic bacteria by 2 logs (US EPA 2004a).
Kabrick and Jewell (1982) found that Salmonella spp. was reduced to undetectable levels in an aerobic reactor at 35 °C in 24 h, while at 60 °C Salmonella spp. was eliminated in few hours. Han et al. (2011) studied the efficiency of anaerobic lagoon fermentation (ALF) and autothermal thermophilic aerobic digestion (ATAD) for removal of pathogenic bacteria in raw swine manure. The results revealed that in raw swine manure, Dialister pneumosintes, Erysipelothrix rhusiopathiae, Succinivibrioan dextrinosolvens, and Schineria sp. were detected. ATAD (autothermal thermophilic aerobic digestion) exhibited more efficiency to eliminate of these pathogens than ALT.
In the mesophilic ALF-treated swine manure, Schineria sp. and Succinivibrio dextrinosolvens were still detected, while were undetected in ATAD. These findings support the superiority of ATAD in selectively reducing potential human and animal pathogens compared to ALF (anaerobic lagoon fermentation), which is a typical manure stabilization method used in livestock farms. In a comparison between aerobic and anaerobic treatment of biosolids, it can be indicated that the anaerobic process is more efficient than the aerobic process, but the limitations to apply the anaerobic treatment lie in the design and the maintenance in the developing countries.
The reuse of sewage-treated effluent and biosolids for agricultural purpose has increased extensively in the last few decades in the Middle East countries. Those countries face a severe shortage of water resources and tertiary sewage treatment plants are not available. Treated sewage and biosolids are rich with nitrogen and phosphate that would improve plant growth and soil properties. However, it has also a large diversity of pathogenic bacteria, which represent a potential risk for human and animal. Most of these pathogens could survive for a long time in the environment, because these pathogens produce endospores and others can survive in VBNC (viable but non-culturable) state.
Therefore, further treatment of sewage effluents and biosolids generated from secondary treatment is necessary to reduce the pathogenic bacteria before reuse for agricultural purpose. Further treatment using technologies such as SODIS, air-drying and lime treatment appears to be more suitable to the nature and climate of the world and can vouch for their efficiencies. These processes have the potential to reduce pathogenic bacteria in treated sewage and biosolids. Economically, the processes are efficient, easily implementable, and natural, with no toxic by-products and most of all are of low cost.