ECONOMIC ASPECTS OF APPLICATION OF PRETREATMENT SYSTEMS BEFORE GRANULAR ACTIVATED CARBON FILTRATION

S t r e s z c z e n i e Granulowany węgiel aktywny (GWA) jest stosowany do usuwania nadmiernej ilości organicznych zanieczyszczeń powodujących niepożądany smak, zapach lub barwę wody oraz mikrozanieczyszczenia, takie jak metale ciężkie lub toksyczne związki organiczne. Jednak sorpcja jest jednym z najdroższych procesów jednostkowych stosowanych w uzdatnianiu wody. Wynika to z wysokiego kosztu zakupu granulowanego węgla aktywnego i konieczności jego wymiany lub regeneracji. W artykule przedstawiono wyniki analizy efektywności ekonomicznej budowy układów wstępnego oczyszczania wody przed procesem adsorpcji. Przeprowadzona analiza pozwoliła na określenie opłacalności tego typu inwestycji warunkowanej przez jakość wody surowej oraz czas pracy złoża sorpcyjnego pomiędzy regeneracjami. Wyniki symulacji pokazały, że praktycznie w każdym analizowanym przypadku opłacalna jest budowa układu wstępnego oczyszczania. K e y w o r d s : Economic effectiveness; Granular activated carbon adsorption; Pretreatment systems. 4/2017 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T 123 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T The Si les ian Univers i ty of Technology No. 4/2017 A . G u m i ń s k i , M . K ł o s , J . G u m i ń s k a 124 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T 4/2017 public demand for safe drinking water, the combination of ozonation and granular activated carbon technology is used to improve the drinking water quality [2–5]. Adsorption on granular active carbon (GAC) or its use as a carrier for immobilization of microorganisms in order to carry out biological water treatment is one of the most widely used processes for removing an excessive amount of organic contaminants causing undesirable taste, odour or colour of water and refractive micropollutants, such as heavy metals or toxic organic compounds [6–10]. However, adsorption is one of the most expensive unit processes used in treatment and renewal of water. This is due to high cost of granular active carbon and the necessity of its frequent exchange or regeneration which stems from its limited adsorption capacity. Adsorption process on activated carbon is one of many processes, which operational and design parameters should be determined in pilot research. It results from the fact that the efficiency of adsorption is affected by many factors related to raw water quality and sorbent properties. The most important is the content of organic compounds and residual coagulant. Taking into consideration the properties of activated carbon, a primary factor limiting adsorption capacity is a pore size of sorbent which should be adjusted to characteristics of contaminants in raw water. The improper choice of activated carbon causes that its effective operation time can be shorten even several times [11]. GAC’s capacity is pH depended. McCreary and Snoeyink (1980) and Weber et al. (1983) demonstrated that as the initial pH was lowered, the adsorption capacity of GAC was improved. It results from a decrease in the solubility of NOM at lower pH [12, 13]. In order to increase the effective time of adsorption, pretreatment processes of water supplied to GAC are commonly used. The most frequently used processes are as follows: – conventional coagulation with sedimentation or flotation, as a separation method of post-coagulation suspensions, – rapid filtration to remove suspensions which could block the specific surface area of sorbent, – ozonation to enhance biodegradability of organic contaminants, which increases the availability of organic carbon as a source of energy for microorganisms immobilized on a GAC surface bed, and changes the mechanism of adsorption into biological filtration. The impact of above-mentioned pretreatment processes on effective operation time between regenerations of GAC is different. The most common process is rapid filtration. It is used to remove suspensions of a size larger than 10 μm, which may block active surface of sorbent and increase hydraulic filtration resistance in a GAC bed. Implementation of rapid filtration as a pretreatment process allows to increase the effectiveness of adsorption, and increase the duration of a GAC filtration cycle. The lack of rapid filtration before GAC filters causes that a GAC bed must be backwashed more frequently than GAC filters which are preceded by rapid filtration. Increased frequency of GAC filter backwashing negatively affects sorbent grains, causing increased abrasion and generate increased heat loss. In addition, if raw water contains suspension with high adhesive properties, e.g. precipitated iron and aluminium hydroxides, it can lead to irreversible clogging of surface of sorbent grains, which cannot be removed during backwashing process. This may cause a significant decrease of treatment efficiency. One of relatively frequently used methods of pretreatment is coagulation and sedimentation or dissolved air flotation. Coagulation applied as pretreatment to GAC can both reduce NOM concentration and decrease the initial pH leading to improvement of GAC performance. Coagulation preferentially removes colloids and large molecular size fraction and humic acid, which could adversely affect the effectiveness and efficiency of active carbon adsorption. A study by Semmens et al. (1986) showed that if GAC is proceeded by coagulation pretreatment, filter-adsorbers have longer run times. It was also demonstrated that further improvements in GAC run time is possible to achieve at higher coagulant doses. Coagulation process allows to reduce total organic carbon (TOC) in the range between 40% and 70%. The experience of the authors shows that the use of coagulation enables the extension of a period between GAC bed regeneration processes from 3–6 months to 18, and sometimes even up to 24 months. Due to limited effectiveness of settlers and flotation chambers, coagulation systems must be supplemented by rapid filters [15–17]. The adsorption capacity of GAC is affected both by NOM concentration and chemical nature of organic pollutants. The most effective way to extend operation time of GAC filters is to apply the initial process of ozonation. This process is aimed at increasing the biodegradability of organic pollutants in raw water and changing the mechanism of sorption from physiECONOMIC ASPECTS OF APPLICATION OF PRETREATMENT SYSTEMS BEFORE GRANULAR ACTIVATED CARBON FILTRATION E N V I R O N M E N T e 4/2017 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T 125 cal sorption into biosorption of refracting contaminants. The application of ozonation, coagulation and rapid filtration in a pretreatment system lengthens operation time of GAC bed by up to 40–50 months. The common form of biofiltration is to operate rapid filters in a biologically active mode. The superiority of BGAC may results from its better attachment surface for biofilm bacteria and being able to absorb some of the input biodegradable dissolved organic carbon or organic materials that are released by microorganisms [18–23]. During biofiltration, pollutants that are present in the water are removed by the processes of adsorption on activated carbon and biodegradation. Activated carbon accelerates the decomposition of ozone into highly oxidative species, such as hydroxyl radicals (HO•) [24]. The biodegradation is a result of the presence of microorganisms on the external surface and in macropores of the GAC [21, 25]. The microorganisms use dissolved organic compounds and previously adsorbed onto the carbon, in their metabolic processes. It results in the recovery of the adsorption capacity of the GAC. Hence, the application of ozonation and BGAC leads to production of biologically stable water [21, 25, 26]. Ozonation and biological filtration as pretreatment before GAC change composition and organic matter structure and hence significantly affect adsorption capacity. Ozonation of NOM causes a change to lower-molecular-weight compounds of lower UV254 absorbance and of higher biodegradability. Biologically active filters can utilize biodegradable compounds, lowering NOM concentration. NOM absorbability usually decreases with ozonation because it creates more polar, hydrophilic compounds. However, biological filtration can compensate for the negative effect of ozonation on adsorption by removing these weakly adsorbing compounds. As a result of ozonation and biological filtration the reduction of organic concentration increases and so the run time of a GAC contractor. The removal in NOM concentration resulting from the ozonation and biological filtration is influenced by an ozone dose and the quality of water being treated. Some researchers have found that ozonation results in a decrease in the formation of THMs and HAAs upon subsequent chlorination, however, a high removal of the THMs precursor cannot be expected by simple ozonation [28–30]. It only makes the organic molecules smaller, more oxidized, and more biodegradable [31–35]. BDOC that is produced by ozonation can be removed in subsequent biological treatments. The ozonation should be set to achieve treatment goals that are based not only on the characteristics of the NOM in the raw water, which optimizes biofiltration, but also on economic benefits [36–37]. Yan et. al (2010) demonstrated that the integrated process of ozonation and BGAC (O3/BGAC) is superior to GAC for the removal of the THMs precursor because a considerable synergetic effect occurs between the ozonation and the BGAC. Although ozonation can limitedly remove dissolved organic carbon, it can cut down the molecular weight of the NOM, change its polarity, decrease the THMFP, and obviously enhance the efficiency of the BGAC. The BGAC could efficiently remove the hydrophobic base, hydrophobic neutral, weakly hydrophobic acid, and low molecular weight fraction DOC that was produced in the optimized ozonation proce