Adsorption and desorption processes of trihalomethanes on different granulated activated carbons in a full-scale advanced water treatment plant

THMs are a group of organic chemicals that contain one carbon atom, one hydrogen atom and three halogen atoms. The most common of these halogen atoms responsible for THM formation in water are bromine and chlorine. THMs are volatile, hydrophobic and non-biodegradable chemicals. Chloroform is typically found at higher concentrations in potable water than any other THM. This type of compounds can be treated by adsorption on granular activated carbon (GAC) (Zietzschmann et al. 2016). As a result, activated carbon filters are increasingly being implemented in full-scale for drinking water treatment (Kasuga et al. 2010). In this respect it should be noted that the more brominated the THM, the higher the adsorption capacity of the activated carbon. GAC has the highest adsorption capacity for bromoform and least with chloroform.

Adsorption of THMs and NOM on GAC has been investigated in depth by different authors (Çapar & Yetış 2002; Babi et al. 2007; Verdugo et al. 2013). However, there are few studies on desorption phenomena of different compounds from GAC during DW treatment (Yapsaklı et al. 2009; Corwin & Summers 2011; Kwon et al. 2017; Aschermann et al. 2019; Reif et al. 2020) and no study has been found on desorption at a real scale reporting data of THMs produced, when the characteristics of the water and/or previous processes are modified. This desorption process is crucial as it could increase the total amount of by-products such as THMs to critical levels.

Desorption process occurs when (1) adsorbed compounds are displaced by more strongly adsorbing compounds, or (2) when the concentration gradient in the adsorber reverses and adsorbed compounds are driven into the water phase by back diffusion (Corwin & Summers 2011).

The public water utility Bilbao Bizkaia Water Consortium (BBWC) planned and built a modular and advanced Drinking Water Treatment Plant (DWTP). This DWTP constitutes the ‘Bilbao Bizkaia Water Treatment Advanced Centre’ whose Spanish acronym is (CATABB) which has a 6.25 m³·h⁻¹ treatment capacity. This advanced DWTP can treat water supplied from different sources using different technologies included GAC filters.

The objective of this study is to evaluate at a full scale, in the DWTP of the CATABB, the adsorption and desorption capacity of THMs from different GAC filters. To carry out this study this water plant was operated during 3 years, and different operational conditions and types of source water have been evaluated.

• 1.

  Pilot advanced DWTP

The pilot-plant experiments were conducted in the advanced pilot DWTP that BBWC built close to Bilbao (in Etxebarri, Bizkaia, Spain) with a 6.25 m³·h⁻¹ treatment capacity.

This pilot plant is fully automated and on-line monitored. It also counts with a laboratory where most of the analyses are performed. Some specific analysis, as well as the validation of the different analysed parameters, has been carried out at the BBWC laboratories. This DWTP has been operating for three years with different water sources and configurations.

• 2.

  Water sources

Water from Zadorra reservoirs was fed into the DWTP. When chlorine was dosed as pre-oxidant (1.5 mg·dm⁻³) an Aplicor equipment was used. In the case of using ozone (1.5 mg·dm⁻³) the equipment was supplied by Newland company.

Aluminium sulphate as coagulant and polydadmac as flocculant, both from Acideka, were used in the settler.

Water from each of the stages of the treatment was analysed twice a day.

• 3.

  Adsorbents

Two commercial GACs, Cabot Norit 1240AF and Calgon (Chemviron) Filtrasorb 400, were used for this research. Properties of both carbons are shown in Table 1.

The characteristics of both GAC filters were the following ones: GAC volume (1 m³), diameter (0.92 m), filtration area (0.67 m²), total height (3.45 m), water flow (6.25 m³·h⁻¹), filtration speed (9.82 m·h⁻¹), backwashing periodicity (800 h).

• 4.

  Analytical parameters

TOC and DOC samples were analysed with a Shimadzu TOC-4200 Analyzer and 0.45 μm Olimpeak (Teknokroma) syringe filters for samples filtration.

Ultraviolet absorbance (UVA) was analysed at a wavelength of 254 nm (UV254) using a WTW PhotoLab 7600 UV-VIS Spectrophotometer. Specific ultraviolet absorbance SUVA (calculated by dividing the UV254 nm by the DOC of a given water sample, expressed in units of L/mg.m) value in water was determined as precursor for the formation of THM during chlorination of water (Marais et al. 2019).

THMs were analysed with a THM-100 system of Instrumentación Analítica company.

Other parameters were analysed at the laboratories of the BBWC using standardised methods and quality assurance protocols.

This section compiles the results of the adsorption and desorption tests carried out in the CATABB water plant. This plant has two GAC filters (1 m³) placed in parallel. In all cases the empty bed contact time (EBCT) was of 9.6 min. This value is higher than the normal value of 7 min used for municipal systems. Longer times up to 15 min EBCT are reported to be beneficial to more fully saturate the GAC with THMs (Potwora 2006). First, adsorption–desorption isotherms were performed by other research group, but results are not publicly available. The main conclusion of this study was that the Chemviron coal performed slightly better than the Norit one.

From August 2018 to January 2021, the two GAC filters with the two types of carbons (Norit1 and Chemviron1) were used alternately with the same type of feed water.

In January 2021 both filters were replaced and two new carbons (Norit2 and Chemviron2), from the same manufacturers, were installed.

In the case of the Norit2 filter, the feed water was doped with chloroform (≅2.0 mg/L), as a source of THMs, to accelerate its saturation process.

From based on the inlet and outlet THM concentrations, as well as the operating flow rates, an analysis of the adsorption capacity (g THM/kg GAC), as a function of the volumes of treated water per kg of GAC, has been carried out to make the following comparisons:

• 1.

  Chemviron1 and Norit1 activated carbon comparative operated in the period from September 2018 to February 2021 in the pilot plant.

• 2.

  Comparison of Norit1 and Norit2 activated carbon, in which the adsorption capacities of Norit1 activated carbon, used in the tests with the different configurations and water origins for two and a half years, versus Norit2 activated carbon, to which high doses of chloroform were applied to achieve accelerated saturation, are evaluated.

• 3.

  Comparison of Chemviron1 and Chemviron2 activated carbon in which the differences in the adsorption and desorption process between the new GAC filter (Chemviron2) and the used one (Chemviron1) are analysed.

• 1.

  Comparison of Chemviron 1 and Norit 1 GAC filters

For these two GAC filters the comparative adsorption tests were performed after treating a similar volume of water. In both cases the source of water was Zadorra reservoirs and first chlorine (1.5 mg/L) was used as pre-oxidant.

When the pre-oxidant agent was changed to ozone (with a dose of 1.5 mg/L), and consequently GAC filters were no longer fed with a THM stream, in both filters the desorption process was observed. DBPs as well as oxidised NOM when ozone is used are different ones than in the case of using chlorine. Consequently, the concentration of THM at the GAC outlet was higher than the concentration at the GAC inlet. In both filters, the desorption of THM occurred gradually, at a rate comparable to that of adsorption. THM concentration at the outlet of the Norit1 filter decreased from 35 μg/L at the end of the adsorption tests (5,000 BV), to 10 μg/L after 3,000 BV and then decreased more slowly (Figure 3). In the case of the Chemviron1 filter, a concentration of 10 μg/L was reached after treating less than 1,000 BV of water and then remained constant at that dose (Figure 4).

The amount of adsorbed and desorbed THMs in both carbons was monitored. For the Norit1 filter, the amount of adsorbed THMs (0.14 g THM/kg GAC) was like the desorbed one. In the case of the Chemviron1 filter, the desorption rate was slower than the adsorption rate, as only 0.05 g THM/kg GAC was desorbed in the desorption stage (3,000 BV) compared to 0.2 g THM/kg GAC in the adsorption stage (4,500 BV). This difference between the two carbons can be attributed to their different characteristics (microporous contribution rate, basicity, surface area).

• 2.

  Comparison of Norit 1 and Norit 2 GAC filters

From the evolution of THM concentration in the adsorption and desorption stages after chloroform doping (Norit2) and from the estimation of the adsorption/desorption capacity the following differences and similarities with respect to the tests performed without doping were identified.

The adsorption process in the chloroform doping tests was faster, the amount of adsorbed THMs and the degree of saturation of the GAC Norit1 filter was higher than for the tests without doping. For Norit2 the THM concentration of the effluent was stabilised after treating 1,000 m³ of water, while without doping (Norit1) (Figure 2) THMs remained constant after 3,000 m³. Norit2 registered similar THM concentrations in the inlet and outlet samples after doping, so it seemed that the charcoal was already saturated. On the contrary, for the Norit1 filter the outlet THM concentration was between 5 and 10 μg/L lower than that at the inlet, so the capacity curve showed an upward trend, reflecting that it was still able to absorb THM.

As for the adsorption capacity, it was estimated that up to 1.8 g THM/kg GAC had been adsorbed during the doping of the Norit2 filter, which represented 15 times more than the value obtained for the no doped filter (Norit1) in which 0.12 g THM/kg GAC were adsorbed. These differences observed in adsorption capacity and rate can be attributed to the contribution of two main factors:

• - A higher degree of pre-saturation of the Norit1 filter which was used for a period of more than 2 years, versus the Norit2 filter which had a usage time of several days prior to the chloroform doped adsorption and desorption tests.

The higher THM concentration in the feed water of the Norit2 filter during the doped tests, according to the adsorption isotherms, resulted in higher adsorption capacities compared to the non-doped Norit1 filter.

As in the desorption tests performed with the GAC filters without chloroform doping, after changing the pre-oxidant type to ozone, for the Norit2 filter the THM concentration in the effluent was also higher than in the feed water (Figure 4). However, during the period of the desorption tests that lasted about one month, and in which about 4,000 m³ of water was treated, the THM concentration at the outlet of the GAC filter increased instead of decreasing, as observed in the non-doped tests, following a different trend with fluctuations and slight increases of the THM values ranging from 33 to 60 μg/L.

These results indicate that even upon adsorption of high amounts of THMs, which occurred during doping, desorption process occurs slowly. In fact, the analysis of the amount of desorbed THMs showed that the desorption rate after doping in the Norit2 filter was comparable to the desorption rate of the Norit1 filter (without doping) and much lower than the adsorption rate during doping.

• 3.

  Comparison of Chemviron 1 and Chemviron 2 GAC filters

In the desorption phase (using ozone as pre-oxidant), while in the case of the used filter (C1), the measured THM concentration in the effluent was between 10 and 15 μg/L, concentrations higher than that obtained for the inlet water. The THM concentration of the effluent water decreased progressively; in the case of the new filter (C2) the THM levels in the effluent remained below 5 μg/L, being only slightly higher than those of the inlet water.

In Table 2, the main characteristics of the feed water, as well as of the effluent, during the research with these two filters are shown.

The main goal of this study was to show real data on the adsorption and desorption processes of THMs in GACFs at a full-scale DWTP operating under different conditions.

At the CATABB full-scale water plant we checked how this desorption process occurred for two different types of carbons, when the pre-oxidant was changed, and ozone was used instead of chlorine. This desorption process could suppose a risk, however, for the different tested GAC filters, the THM desorption process was rather slow as the asymptote is reached after about 4 weeks of operation. The obtained constant value of THM after the GAC filter was quite different depending on the type of GAC as well as the previous usage, the filter. In addition, no peaks of THM levels or concentrations of concern for desorbed THMs were detected even when the inlet water was doped with high doses of THMs (2.0 mg/L). Regardless of the desorption process, the THM concentration of the treated water in the CATABB plant was always far below the maximum established limit of 100 μg/L (ED 2020/2184).

The adsorption capacity of GAC varied widely, depending on the quality of the source water and the pre-treatment given to it. Many factors can reduce this capacity such as: organic matter, preloading of organics onto the carbon, temperature, pH, variable influent concentrations, type of pre-oxidant, and adsorption kinetics.

Data cannot be made publicly available; readers should contact the corresponding author for details.

The authors declare there is no conflict.