The Role of Selected Technological Processes in Drinking Water Treatment

Water treatment plants face challenges related to the selection of the optimal water treatment technology which has a significant impact on human health and on maintaining the balance of the natural environment. The observed population growth, lower availability of natural freshwater resources and deterioration of its quality are caused by climate and hydrological changes, land use and emerging new environmental pollutants. The list of compounds referred to as emerging pollutants (EP), contaminants of emerging concern (CEC), micropollutants (MP), priority pollutants (PP), persistent toxic substances (PTS) or substances of very high concern (SVHC) is constantly growing [1]. These substances pose a threat to human health and are persistent in the natural environment [2]. These facts justify the urgent need to develop and apply highly efficient and versatile water treatment processes capable of coping with the changing composition of natural organic matter (NOM) and its increasing concentration in the aquatic environment [3]. High values of turbidity, color, planktonic organisms, bacteria and viruses, as well as dissolved organic carbon most often determine the choice of water treatment methods [4].

The removal of turbidity and NOM usually involves the use of several processes, not just one. So far, a number of technological processes have been tested to remove turbidity, NOM, metals and metalloids from water, such as coagulation, flocculation, sedimentation, filtration on sand beds, membrane filtration, oxidation and adsorption processes [5]. These methods have their advantages and disadvantages. The use of inorganic coagulants such as aluminium and iron salts removes turbidity and only a part of NOM. The removal of NOM by means of flocculation, oxidation and membrane processes does not always yield the expected efficiency, it is energy-intensive, consumes a large number of chemicals and generates waste [6]. In contrast, coagulation, flocculation, sedimentation/flotation and filtration processes are considered the most economical solutions to remove NOM. Coagulation removes most of NOM in the form of hydrophobic fractions having high molecular weight and to a lesser extent hydrophilic fractions of low molecular weight [7]. In turn, adsorption with granulated activated carbon is one of the best methods of removing NOM (including the hydrophilic fraction) [8]. Also other adsorbents used to remove NOM were tested, e.g. powdered activated carbon (dust), modified carbon, single and multilayer carbon nanotubes, particles of iron oxide (ferrihydrite, goethite, hematite) and magnesium [9], iron oxide-coated sand [10], chitosan, powdered activated carbon composite, polypropylene-coated glass, beads, aminated polyacrylonitrile fibres, chitosan/zeolite composite, rice husk ash, surfactant-modified bentonite, resins (MIEX®) and others [11]. The processes comprised in MIEX®DOC include adsorption, coagulation and ion exchange [12].

The presence of plankton, bacteria and viruses in water also justifies the use of coagulation processes assisted by pre-oxidation, filtration and adsorption as well as disinfection. From the viewpoint of water treatment technology, planktonic organisms that are inactive after the oxidation process have properties similar to those of colloid particles [13]. Microorganisms are made of polar particles, and when lifeless they behave similarly to hydrophilic colloids. The research conducted by [14] demonstrated the high efficiency of coagulation in removing cyanobacteria from water. Researchers also pointed out the need to optimize wastewater treatment processes [15] and the management of post-coagulation sludge (PACl and AlCl₃) [16] and sewage sludge [17,18], due to the possibility of releasing cyanobacterial microcysts into the environment [14]. Coagulation conducted as a stand-alone process removes organic matter from water to a small extent, measured as dissolved organic carbon (DOC) and ultraviolet absorbance (UVA_254nm). This is because coagulation is effective in removing medium to large DOC molecules. And coagulation combined in the right order with adsorption and/or ion exchange is characterized by high efficiency in the removal of organic pollutants [19].

Organic substances of natural origin tend to form strong organometallic complexes with heavy metals, which have wide bioavailability and high toxicity, and they are also precursors of oxidation and/or disinfection by-products [3]. These products will appear in water when strong oxidants such as chlorine (I) and chlorine (IV) oxide are used for disinfection and/or oxidation in water treatment processes. About 500 by-products of oxidation and/or disinfection are known, whereof about 20% are aliphatic trihalomethanes (THM) and 10% are halogenoacetic acids (HAA), haloacetonitriles (HAN), haloketones and trichloronitromethane along with numerous aromatic halo-DBPs [20].

Water treatment with the use of artificial infiltration is a natural and reagent-free purification process, based on the use of a ground layer as a multifunctional reactor in which chemical, physical and biochemical processes take place. With the current shortage of groundwater and the constant lowering of its level, infiltration should be used more widely [21].

The main research goals set by the author were: 1)

Analysis of water treatment processes used in WTP in Stary Sącz in terms of surface and underground water treatment.

The efficiency of water pollutant removal at individual stages of the treatment process at the WTP is presented in Table 1.

Technological processes of water treatment at the WTP should be selected so that in time periods of extreme, above-standard values of water quality parameters before treatment, they were able to purify water to the normative values specified in the Regulation of the Minister of Health [22]. In the research work, it was raw water collected from the Dunajec River that was first analyzed. In the studied period, the waters of the Dunajec River were characterized by high dynamics of the variability of physical, chemical and bacteriological parameters (Table 1). Sudden, raised values of turbidity parameters (max. 2550 NTU), color (max. 964.8 mgPt·dm⁻³), UVA_254nm (max. 4120), permanganatate oxygen demand (max.10.0 mg·dm⁻³), total iron (max. 2.480 mg dm⁻³), manganese (max. 0.0438 mg·dm⁻³) were observed in spring and summer (during thaws, storms, sudden outflows of streams or floods). The bacteriological contamination of the Dunajec was determined by the number of coli group bacteria (max. 3468 cfu·100 cm⁻¹), Enterococcus faecalis (max. 2000 cfu·100 cm⁻¹) and Clostridium perfringens (max. 1000 cfu·100 cm⁻¹) (Table 1).

Then, water quality parameters were analyzed in terms of their removal at individual stages of the treatment process. The leading parameter of the ongoing control of technological processes in the selected WTP was turbidity (Figure. 2), the removal of which in the primary settling tanks of volumetric coagulation ranged from 0.0% to 99.6% with an average of 42.4%. The achieved turbidity values after settling tanks are minimum 0.6 NTU, on average 5.0 NTU and a maximum of 133 NTU. Turbidity removal on Dyna Sand filters ranged from 63.3% to 99.6% with an average of 93.2% (min. 0.09 NTU, average 0.22 NTU, max. 0.81 NTU). At the stage of ozonation and carbon filters, turbidity was additionally reduced by an average of 19.0% (min. 0.1 NTU, average 0.2 NTU, max. 0.5 NTU) and by 39.2% (min. 0.07 NTU average 0.10 NTU, max. 0.21 NTU) (Figure 2). The results of the research confirm that the processes responsible mainly for the removal of turbidity in the WTP technological system is volumetric coagulation in primary settling tanks and filtration on sand filters. Also noteworthy is the process of ground infiltration [23], in which turbidities were reduced with high efficiency, comparable to rapid filtration (min. 74.3%, average 96.9% and max. 99.9%) for the minimum values of 0.6 NTU, 0.12 NTU, 0.85 NTU.

Figure 2.

All natural or surface waters, and to a lesser extent underground waters, contain a wide spectrum of organic compounds. A mixture of these compounds is referred to as natural organic matter (NOM) [7]. The range of NOM components may vary seasonally, e.g. when subjected to the impact of rain, snow, flood or drought, which was also observed in the five-year analysis of the Dunajec River waters (Table 1). Extreme weather conditions such as drought or flood are the main cause of changes in the quality and availability of water, and they contribute to the concentration of NOM. In addition to variable composition, the rise of NOM concentrations in freshwater and municipal wastewater has also been observed over the last 20 years. Due to the risk involving the development of oxidation and disinfection by-products, it is important to remove NOM at the water treatment stage before the chemical disinfection processes.

The color of surface waters is mainly caused by humic substances that penetrate the waters as a result of extraction from the soil, or due to the natural decomposition of plant or animal organic matter (humication) [24]. In the tested technological system, color was removed in primary settling tanks supported by volumetric coagulation and sedimentation with an average efficiency of 20.7%; on Dyna Sand filters supported by contact coagulation the efficiency was higher, and its average efficiency was 69.7% (Figure 3). The efficiencies of residual color removal at the stage of ozonation and filtration on the activated carbon bed averaged 41.1% and 48.4%, respectively, with maximum values of 84.6% and 98.2%, respectively (Figure 3).

Figure 3.

Another parameter used for the ongoing control of NOM content at the WTP in Stary Sącz is the measurement of ultraviolet absorbance (UVA_254nm) (Table 1, Figure 4), which was tested 517 to 869 times in the last 5 years. After the primary settling tanks, the UVA_254nm values ranged from 0.123, an average of 0.302 to a maximum of 1.450 (with heavy rainfall). The removal efficiency ranged from an average of 14.9% to max. 91.3%. After Dyna Sand filters, the obtained absorbance values were from 0.070 to 0.257, with an average of 0.140 (average removal 52.5%, max. 93%). After ozonation, the obtained parameters were in the range of 0.047–0.193 with an average removal of 22.5% and max. 100%, and after carbon filters the values were 0.000–0.126, respectively, with an average removal of 11.1% and max. 95.8%.

Figure 4.

As an indicator of the level of organic compounds in water, also permanganatate oxygen demand is used, whereof content in water intended for human consumption is 5.0 mg·dm⁻³ according to the currently applicable standards [22]. The removability of permanganatate oxygen demand was on average 16.2% in pre-coagulation settling tanks, max. 100%, on average 45.6% on sand filters and max. 89.3%, for ozonation on average 31.6% and max. 100%, and for filtration on carbon filters on average 11.2% and max. 100%. The process of ground infiltration was highly effective in reducing oxidizability – on average 65.8% and max. 100% (Figure 5).

Figure 5.

In the collected water samples, the analyses of total iron and manganese were also carried out, the content of which in raw water could suggest the presence of these compounds in water subjected to treatment processes (Table 1). The test results of Fetotal and Mn content after coagulation and filtration processes on sand filters bespeak of very good removal of the metals, i.e. on average 86.1% for Fetotal and 96.4% for Mn (Figure 6, Figure 7). The removability after carbon filters is also high and averages 40.9% for Fetotal and as much as 98% for Mn (Figure 6, Figure 7). In the case of ground infiltration, the effectiveness was also high and a similar percentage of removal and similar final parameters were obtained as in the case of high-efficiency processes used at the WTP. The average removal was 92.2% for Fe and 99.4% for Mn. The natural processes of oxidation and reduction taking place in the layer of the biological slow filter, which is the ground layer, are characterized by very high efficiency of water purification.

Figure 6.

Figure 7.

The results of bacteria removal analyses in the WTP technological system are presented in Table 2. After pre-coagulation and filtration on Dyna Sand filters, coli group bacteria, Enteroccocus faecalis and Clostridium perfringens were removed with an average efficiency of 91.9% to 97.5%. After ground infiltration, the average removal rate of all types of the tested bacteria was high and amounted to 99.9% each time. The disinfection with UV rays and chlorine gas yielded 100% removal of microorganisms in the water treated at the WTP.

Both high-efficiency technological processes and ground infiltration are excellent barriers to periodically raised parameters of turbidity, color, UVA_254nm, permanganatate oxygen demand, metals and metalloids. Each time, at the dynamically operating WTP in Stary Sącz, the tested samples of the treated water met all EU and national legal standards for water intended for human consumption and were within their lower limits.

The carried out studies and analyses have demonstrated that the technological processes introduced at the WTP in Stary Sącz are appropriately matched to the variable water quality of the Dunajec mountain river. Each time, the results of the tests of water treated at the WTP yield the results close to zero or equal to zero, which bespeaks their high efficiency. The effectiveness of the processes of coagulation, flocculation, sedimentation and filtration on sand beds in removing turbidity, larger fractions of NOM, partly metals and metalloids from water is confirmed by researchers [5, 7].

The pre-treatment processes with the filtration process and the oxidation process through ozonation effectively reduce the above-standard parameters of raw water and they additionally support the subsequent removal processes of colloidal particles, smaller organics, metals and metalloids on activated carbon and microorganisms (disinfection processes with UV rays and chlorine gas). Researchers Hoslett et al. [8] state that adsorption with granular activated carbon in combination with oxidation is one of the best methods of removing NOM (including the hydrophilic fraction).

The high technological regime for conducting preliminary processes (high percentage efficiency of turbidity and color removal) at the stage of volumetric coagulation settling tanks, Dyna Sand filters and preliminary ozonation processes, additionally contributes to a long-term operation of carbon filters, whose period of correct operation without regeneration has been extended from approx. 4 to 6 years (first regeneration after 6 years).

The results of the tests and analyses of the technological systems also allowed for the assessment of the operation of the infiltration pool system with ground infiltration, which can successfully compete with other high-efficiency processes used in the WTP. Kowal & Świderska-Bróż [21] confirm the high effectiveness of infiltration in removing physico-chemical and bacteriological indicators from water. They state that infiltration in the ground is a natural method of water treatment, which, in the current widespread shortage of water resources, should be used more often [21].

The processes of double disinfection with UV rays and chlorine gas each time ensure 100% certainty involving the removal of microorganisms from water subjected to the processes. The primary processes of volumetric coagulation and filtration on a dynamic bed with contact coagulation in Dyna Sand filters together with the processes of ozonation with the filtration on activated carbon effectively prepare water for the final stages of disinfection by removing turbidity and NOM from it. Only water free of turbidity and organic matter can be effectively subjected to physical disinfection with UV rays and chemical disinfection with chlorine gas. Particulate-free water with low turbidity increases the penetration efficiency of UV rays, which effectively inactivate genetic material unable to continue replication and/or kill bacteria and viruses. Reducing the content of organic matter in water subjected to disinfection processes affects both the effectiveness of the process and reduces the formation of disinfection by-products such as THMs.