Manganese removal processes at 10 groundwater fed full-scale drinking water treatment plants

Manganese (Mn) removal in drinking water filters is facilitated by biological and physico-chemical processes. However, there is limited information about the dominant processes for Mn removal in full-scale matured filters with different filter materials over filter depth. Water and filter material samples were collected from 10 full-scale drinking water treatment plants (DWTPs) to characterise the Mn removal processes, to evaluate the potential use of enhancers and to gain further insight on operational conditions of matured filters for the efficient Mn removal. The first-order Mn removal constant at the DWTPs varied from 10 2 to 10 1 min . The amount of Mn coating on the filter material grains showed a strong correlation with the amount of iron, calcium and total coating, but no correlation with the concentration of ATP. Inhibition of biological activity showed that Mn removal in matured filters was dominated by physico-chemical processes (59–97%). Addition of phosphorus and trace metals showed limited effect on Mn removal capacity, indicating that the enhancement of Mn removal in matured filters is possible but challenging. There was limited effect of the filter material type (quartz, calcium carbonate and anthracite) on Mn removal in matured filters, which can be relevant information for the industry when assessing filter designs and determining returns of


INTRODUCTION
Drinking water treatment from groundwater sources in Denmark is based on aeration and biofiltration (single or double stage). Bacteria play an essential role in the removal of ammonium (NH 4 ), while iron (Fe) may be removed chemically following aeration. Removal of manganese (Mn) involves both biological and physico-chemical processes, but knowledge regarding the role of these mechanisms is very limited in fully matured, full-scale filters (Sahabi et al. ; Bruins et al. ; Breda et al. ).
Recent studies suggest that after a short initial sorption phase, Mn removal by a non-coated virgin filter material is initially biological, evolving to a predominantly physico-chemical removal process over time (Sahabi et  The changes observed in the Mn removal process over time (Sahabi et al. ; Bruins et al. ) suggest that the filter maturation period continues after the end of the start-up period. Full maturation is a state in which only limited variation over time occurs with respect to the physicochemical and biological aspects of the surface of the filter material coating, e.g., surface area and charge, surface composition and microbial community (abundance, diversity and activity). The duration of the period between completed start-up and full maturation is unknown and likely dependent on operational conditions and source water quality.
The present study contributes to the overall knowledge of drinking water filters by collecting both water and filter material samples over depth at 10 full-scale DWTPs with different stages of maturation. This study aimed to characterise the Mn removal processes (biological or physicochemical) occurring at different filter depths and to evaluate the potential use of TM and phosphorus to enhance the Mn removal capacity of matured filters. In addition, this study provides practical advice that can be used by water utilities to manage operations and to obtain efficient Mn removal, such as the role of filter material coating and the importance of filter material selection.

Sampling at 10 DWTPs
This study investigated 10 DWTPs treating groundwater in Denmark: five located in Aarhus and five located in Odense (Dalum,Elsted,Holmehaven,Hovedvaerket,Kasted,Lindved,Lunde,Stavtrup,Truelsbjerg and Østerby DWTP,. Location and multivariate analysis of the groundwater quality feeding each DWTP is available in the Supplementary Materials (available with the online version of this paper). The effluent water of each DWTP complies with the drinking water national criteria (BEK  ).
Mn was efficiently removed to below the national criterion of 0.05 mg/L at all DWTPs over the last 20 years.
The removal of iron (Fe), NH 4 and Mn by the DWTPs included in this study occurs in the first filter. In the present investigation, water and filter material samples were collected from the first filter of each DWTP before backwash, i.e., at the end of the ripening cycle of the filter.
Water samples (100 mL) were collected at the filter's inlet, every 10 cm within the filter bed and at the filter's outlet. Collection of filter bed water samples was accomplished by a multiple-depth sampling device with separate screens every 10 cm, which was lowered into the filter bed during backwash and left in the filter until the following backwash. Water samples were withdrawn from the screens with a multichannel peristaltic pump (Cole Parmer, USA).
Samples for analysis of chemical parameters were collected after pH, redox, oxygen (O 2 ) and temperature levels were stable in the withdrawn water. Inline measurements of pH, redox and temperature were recorded using a digital pH electrode (SENTIX940), a redox sensor (ORP-T900), an optical dissolved O 2 sensor (FDO 925) and a Multi 3,430 meter (WTW GmbH, Germany). Redox measurements were corrected to temperature conditions of 10 C. Filtered Treated water (15 L) was collected at each DWTP to use in batch experiments (described later in this section).
Filter material was sampled after draining of the filters overnight. Filter material samples (approximately 500 g) were collected from the filter bed over 20 cm depth intervals. This was accomplished with a sampling device constructed with a 50-mm PVC pipe lowered into the sand bed and a shop vac to lift the sand through the pipe and into a 500 mL blue-cap sampling flask, after separating sand and air/water in a hydrocyclone. After the pipe was lowered 20 cm, the flask was replaced with a new one, and the pipe was lowered another 20 cm.

Characterisation of matured filter material
The filter material total coating mass was calculated by subtracting the mass of the grains before and after acid digestion and drying. Acid digestion was conducted at room temperature in triplicates using approximately 2 g of dried filter

First-order removal constant and removal capacity
The removal constant was calculated for each full-scale filter at each 10 cm depth interval assuming the first-order removal, as follows: where k is the first-order removal constant (min À1 ), [C in ] and [C out ] are the total concentration of C in water samples collected at the top and bottom of each depth interval (mg/L), and t is the empty bed contact time (EBCT) at each depth interval (min), which can be calculated as follows: where depth interval is the distance between sampling ports placed across the full-scale filter depth (0.01 m) and v is the filtration rate of the full-scale filter (m/min).
The removal capacity R c of the filter material (μg/min/g) was determined through the batch assays (previously described). The constant k (min À1 ) was determined as the slope of the first-order regression of C (mg/L) over time t (min) by replacing C in and C out with C initial and C final , respectively in Equation (1). R c was then calculated by multiplying k (min À1 ) by the quotient of the initial mass of the substrate m sub (μg) and the mass of the filter material sample used in the batch m fm (g), as follows: percentile, respectively). The correlation coefficients between elements present in the coating of the filter grains were determined using the Pearson method (Figure 2(a)).
Mn present in the coating showed a strong positive correlation with total coating, Fe, Ca and P but no correlation with adenosine triphosphate (ATP) concentration (Figure 2(a) and 2(b)). The lack of correlation between ATP and Mn does not preclude that biological activity has no effect on the presence of Mn in the coating of the grains since ATP is influenced by many microorganisms in addition to Mn oxidising bacteria (MnOBs).

Effect of the biological inhibitor NaN 3 on the Mn R c
The Mn R c was determined in batch assays using filter material collected over 20 cm depth intervals of each filter.
Even though there was a change in filter material type through depth at some of the DWTPs (Figure 1(a)), the filter material type showed no significant effect on the Mn with the online version of this paper). Thus, the selection of the filter material type can be important during the initial stages of the filter maturation, more specifically during the start-up period. In contrast, the present investigation suggests that there is no effect of the filter material type on Mn R c after a maturation time of 3 years (which is the age of the youngest filter, DWTP1, Figure 1(a)). This information is of interest to the industry for assessing the filter design and determining the return of investments. Mn removal attained by filter material samples without NaN 3 was assumed to be due to biological and physicochemical processes, whereas Mn removal attained by filter material samples with NaN 3 was assumed to be mainly due to physico-chemical processes alone.
In the present study, the Mn R c was statistically lower in the presence of NaN 3 (p < 0.05, Mann-Whitney test).
Mn removal by physico-chemical processes dominated in most filters, representing approximately 77% of the Mn R c of the matured filter material (Figure 3(a)). The effect of NaN 3 for each DWTP (average of triplicates over depth) ranged from 7% to 30% (DWTP5 < DWTP1 < DWTP8 < DWTP9 < DWTP2 < DWTP4 < DWTP10 < DWTP6 < DW TP7 < DWTP3). As all the 10 DWTP were efficiently removing Mn, the results suggest that dominance of biological removal of Mn is not required to attain efficient removal of Mn in full-scale filters with over 3 years of maturation.  in filter material samples collected at 120-140 cm filter depth from DWTP4 and DWTP8, by a factor of 2.4 and 1.2, respectively (Figure 3(b)). These results can be relevant for the water utilities managing DWTP4 and DWTP8, as the respective Mn removal constant k at both DWTPs were among the lowest (Figure 1(c)).
Regarding the overall effect of the potential enhancers on NH 4 , results showed that the NH 4 R c was statistically greater when P and P þ TM were added (p < 0.   (Figure 1(b) and 1(c)). This suggests that Mn removal efficiency is not always dependent on the compliance of the 85% NH 4 removal minimum established by Bruins et al. ().
In the present study, the concentration of Mn and NH 4 measured in water samples collected over filter depth clearly showed that the Mn criterion was often met before the 85% NH 4 removal was reached (Figure 4(a)). More specifically, half of the observations with Mn below the criterion are found in samples with 60% NH 4 removal (Figure 4(a)). In addition to the observations, in situ batch tests using    and Mn removal to Fe, Mn and NH 4 removal.
On a practical note, it is important to clarify that the end of the start-up period is not the end of the filter's maturation (Ramsay et al. ). During the start-up period, virgin filter material matures into a functional filter that can remove substances to drinking water criteria. However, the maturation of the filter continues, changing not only the category of the removal processes occurring in the filter (e.g., Mn removal from biological to mostly physico-chemical) but also the stratification of the removal of specific substances over the filter depth (e.g., Mn and NH 4 removal, Figure 4(b)).

CONCLUSIONS
After characterisation of Mn removal at 10 full-scale DWTPs efficiently removing Mn, this study concludes the following: • The first-order removal constant for Mn in 10 fully matured DWTPs varied from 10 À2 to 10 À1 min À1 . •