In a component of an extensive pilot distribution system (PDS) study, the effects of four different water qualities on biological stability in distributed water were investigated through identical (parallel) single-pass pipe arrangements. Through 24 months of monitoring, a number of key observations were made. Incorporation of a biological treatment step reduced the overall dissolved organic carbon (DOC) loss through the PDS by reducing biodegradable DOC (BDOC) within the water prior to distribution. In the absence of chlorine residuals, the proliferation of culturable organisms was favoured with considerably higher heterotrophic plate counts in samples at the outlet of the PDS. Despite different bacterial cell counts (measured by flow cytometry) entering each PDS from the four treatment streams, equivalent outlet cell numbers were achieved in all systems after 8 months' operation; however multi-step treatment streams took longer to reach equilibrium.
Water quality deterioration in drinking water distribution systems (DWDS) has become a key focus in recent years for many water utilities and service providers. There is agreement amongst water utilities that the aim should be to provide high quality water at the customer tap, while in reality the goal is commonly rationalised to a more achievable target of providing high quality water leaving the water treatment plant (WTP) (Liu et al. 2013). Unfortunately water quality deterioration can occur with increasing water age following detention within a distribution system. This perception is expanding rapidly with a better understanding and recognition that the DWDS has previously been largely considered a chemical and microbiological ‘black box’. The availability of more sophisticated instrumentation has allowed greater insight into DWDS, with a greater focus on the distribution systems as a dynamic rather than static infrastructure component (Kerneïs et al. 1995). Recent investigations into DWDS particulate characterisation (Vreeburg et al. 2008) and microbial ecology (Douterelo et al. 2013) are yielding greater knowledge into the nature of the DWDS as bioreactors, and propagators of sediment accumulation and release. Despite this, standard water quality analyses in the laboratory such as colour and turbidity are still the predominant mode of monitoring DWDS quality, with bacteriological quality based upon chlorine residual maintenance and minimising heterotrophic plate counts (HPCs). Rapid bacteriological assessment tools such as flow cytometry (FCM) (Hoefel et al. 2003; Berney et al. 2008; Hammes et al. 2008) and adenosine triphosphate (ATP) measurement (Van der Kooij et al. 1995; Delahaye et al. 2003; Hammes et al. 2010) are being more widely applied in treatment and distribution system research. These rapid assessments have not yet been implemented into routine monitoring regimes by water utilities.
Due to risks involved in performing bacteriological experimentation on real DWDS, a considerable number of investigations have been reported using pilot facilities to study distribution system behaviour. The most commonly referenced is the Thames Water-developed ‘TORUS’ continuous looped pilot distribution facility (Holt et al. 1994; Smith et al. 1999; Maier et al. 2000; Boxall & Saul 2005). In addition a number of other investigations have examined the effect of water quality, nutrients and flow on the bacteriological stability of pilot distribution systems (PDS) (Piriou et al. 1998; Volk & LeChevallier 1999; Frias et al. 2001; Lehtola et al. 2006; Liu et al. 2013).
While there have been numerous past pilot plant investigations considering biofilms, few have simultaneously studied the effects of multiple different water qualities on biological stability in distributed water through realistic pipe dimensions and materials using identical (parallel) distribution systems over long term operation (24 months). This paper evaluates the change in microbiological enumeration using traditional and next generation techniques before and after passage through the distribution systems and the relationship to treatment methodology.
MATERIALS AND METHODS
The source water for the study was River Murray water taken from the Mannum to Adelaide pipeline at Mt. Pleasant WTP, located in the Adelaide Hills approximately 60 km from Adelaide. The feed waters for the four distribution systems were of increasing product water quality resulting from four different treatment streams in line with current advanced processes utilised within Australia. Source and feed water quality has been described extensively in previous publications (Fabris et al. 2013, 2015).
S1 – Conventional (Conv) comprised alum coagulation followed by flocculation, sedimentation and dual media (sand/anthracite) filtration. Aluminium sulphate was used as the primary coagulant at dose rates dependent on water quality with pH controlled between 6.2 and 6.5 using either sodium hydroxide or sodium bicarbonate, depending on the source water alkalinity. A cationic poly-acrylamide LT22 (BASF Chemicals, Australia) was dosed as a flocculant aid.
S2 – MIEX plus Coagulation (MIEX/Coag) consisted of pre-treatment using a magnetic ion-exchange resin (MIEX) for dissolved organic carbon (DOC) removal, coupled with coagulation/sedimentation/filtration treatment as a clarification step for turbidity reduction. The MIEX–DOC® removal process (IXOM, Australia) implemented at Mt. Pleasant WTP has been described in detail previously by Drikas et al. (2011).
S3 – MIEX plus Coagulation plus GAC (MIEX/Coag/GAC) comprised the product water from MIEX/Coag with further polishing by granular activated carbon (GAC). Two gravity fed filter columns filled with F400, a coal based steam-activated GAC (Calgon Corporation, USA), were used to achieve an empty bed contact time of approximately 14 minutes with the product streams combined.
S4 – Nanofiltration with Microfiltration pre-treatment (MF/NF) incorporated dual stage membrane filtration with a Siemens–Memcor submerged microfiltration (MF) pre-treatment for particulate removal followed by a DOW–Filmtec NF270 nanofiltration (NF) membrane for organics removal and hardness reduction. This stream represented the most advanced treatment technology and consistently achieved the highest treated water quality.
All treated water streams were disinfected to meet a minimum ‘Chlorine concentration x contact time’ factor – Ct (Baumann & Ludwig 1962; White 1975) of 30 mg.min/L, according to demand but deliberately controlled to retain no residual at the inlet to the PDS following 4 hours contact in the treated water storage tank. This strategy was chosen to replicate low-flow ends of distribution systems, where disinfectant residual is often lost and to encourage more rapid establishment of any potential biofilms within the operational duration (24 months). Treated water storage tanks were modified 1,000 L HDPE intermediate bulk containers with treated and disinfected water entering through a ported lid and exiting to the distribution systems via the drain valve.
Pilot distribution systems
Grab samples for DOC analysis were filtered through 0.45-μm pre-rinsed membranes and measured using a Sievers 900 Total Organic Carbon Analyser (GE Analytical Instruments, USA).
Biodegradable dissolved organic carbon (BDOC) was measured according to the method of Joret & Levi (1986). Briefly, the inoculum was biologically active sand (sand colonised by bacteria) originating from a local drinking WTP filter. A 900 mL water sample was inoculated with 300 g of sand and aerated for the duration of the experiment. DOC was measured at the beginning and then approximately every second day until a minimum value was reached (approx. 10–12 days). BDOC concentration is derived from the difference between the initial and minimum DOC values.
Biofilm formation potential monitors (KWR, The Netherlands) based upon an upflow column filled with 12.4 cm2 surface area glass coupons were employed (Van der Kooij et al. 1995). Biofilm coupons were sampled aseptically using a pre-flamed stainless steel wire hook into sterile 30 mL Eppendorf tubes containing 10 mL of autoclaved tap water. Sample tubes containing the glass coupon were ultra-sonicated in a water bath for 10 minutes, then decanted into another sterile tube. An additional 10 mL of autoclaved tap water was added and the procedure was repeated twice more with all solutions combined to obtain a composite biofilm solution. This was centrifuged at 4,500 relative centrifugal force (rcf) for 30 minutes then the supernatant was removed and the pellet was vortexed to resuspend. ATP concentrations were determined according to the method of Hammes et al. (2010). A commercial bacterial kit (BacTiter Glo, Perkin Elmer, USA) was applied with ATP calibration standards of 1 × 10−6, 1 × 10−7, 1 × 10−8, 1 × 10−9 and 1 × 10−10 M. Due to the constraints of running alongside a full-scale WTP, only one continuous 140-day assessment could be made between February and June 2012.
HPCs and FCM
Bacterial enumeration was conducted using both traditional HPCs and FCM, an advanced laser optical technique. HPCs were performed in accordance with the Australian Standard AS/NZS 4276.3.1 (Australian Standard, 1995) using R2A solid media (Oxoid, Australia). Dilutions, when necessary, were performed in maximum recovery buffer (0.1% (w/v) neutralised bacteriological peptone (0.85% (w/v) NaCl, pH 7.0). Incubation was performed using standard conditions of 20 °C for 72 hours. Results for HPC were presented as colony forming units per mL (CFU/mL). HPC samples were taken fortnightly throughout the operational period. FCM analysis was conducted using a FACSCalibur flow cytometer (Becton Dickinson, USA), emitting at a fixed wavelength of 488 nm. Bacteria were enumerated following staining of the bacteria with SYTO-9 and propidium iodide (BacLight™ bacterial viability kit, Molecular Probes, USA) as described previously (Hoefel et al. 2003). Bacterial enumeration data were processed to determine similarity of inlets and outlets across all four streams using mathematical cluster analysis, visually presented as dendrograms, with separation expressed by Euclidean linking distances representing the dissimilarity of datasets if they were spatially positioned (‘R’ version 2.15.1, R Core Team 2012).
RESULTS AND DISCUSSION
Distribution system bacteriology
Treatment efficiency link to biofilm potential
In addition, biofilm potential monitors were installed towards the end of the investigation period to evaluate the capacity for new biofilm establishment in each of the four treated waters prior to disinfection and entry to the PDS. After a 45-day lag period, biofilm activity in the Conv stream biofilm potential monitor increased, while the other three streams exhibited a lack of biofilm growth, expressed through low metabolic activity from the removed glass coupons (low/zero ATP). This suggests that at the time of the application of the biofilm monitors, multi-stage and advanced treatments were creating more restricted and selective conditions for biofilm proliferation (Figure 7(b)). This is consistent with the findings of Liu et al. (2013), who found that a tight membrane treatment (NF) was more effective at controlling biofilm formation than ion exchange treatment and loose membrane treatment (ultrafiltration). Since all treatment technologies applied except for the NF were ineffective for reduction of inorganic nutrients, this implies that they were not growth limiting. In a pilot study, Frias et al. (2001) showed that addition of nitrogen, phosphorous and sulphur did not contribute to greater growth of bacteria; however the organic nutrients were critical to the proliferation or lack of bacterial growth. The relationship of biological activity to organic carbon was also seen in this water source where the treatments that reduced bulk DOC more effectively were more successful in slowing the development of biological activity in the biofilm potential monitors. In addition, the MIEX/Coag/GAC stream, which incorporated a biological treatment through the GAC filter, showed the lowest level of biological activity, suggesting that AOC was also reduced and more efficiently than even the lower DOC MF/NF stream.
PDS bacteriology link to water quality parameters
In most literature sources it has been shown that there is poor correlation between bacterial enumeration in distribution systems and traditionally monitored water quality parameters (Power & Nagy 1999; Carter et al. 2000). In most cases this is because water quality parameters are often present at orders of magnitude greater than what is required to limit or control the rate of growth; therefore the relationships rely on a degree of covariance rather than true correlation (varies together with a parameter, not because of the parameter). In addition, for many water quality parameters that represent growth factors for biological activity, the relationship is causal, with the change producing a growth response only after a lag period during which the composition of the natural biota may shift to favour the species which are best adapted to the new water quality. Regardless, the use of easily measured surrogate parameters is still very appealing. One of the major growth factors controlling bacterial growth is organic carbon. While analyses like BDOC and AOC may be more directly relevant to bacterial regrowth and can help explain changes that occurred in past operation (Huck 1990; Escobar & Randall 2001; Volk & LeChevallier 2002), the lengthy period required to undertake these analyses limits their operational usefulness. Although DOC can be viewed as a bulk parameter containing both biodegradable and refractory organic matter, the change in DOC through the PDS may still be relatable to the potential for the water to support biological activity. All PDS experienced both losses and increases at different times throughout monitoring. DOC measurement before and after passage through the PDS, averaged over 26 months, showed that 0.29 mg/L (Conv); 0.22 mg/L (MIEX/Coag); 0.06 mg/L (MIEX/Coag/GAC) and 0.03 mg/L (MF/NF) of the average DOC that was input was lost, likely through incorporation into cell biomass and/or mineralisation. This represents 7.5%, 9.4%, 3.7% and 5.9% of the inlet DOC for Conv, MIEX/Coag, MIEX/Coag/GAC and MF/NF, respectively. The smallest normalised loss through the distribution system was in the MIEX/Coag/GAC stream, due to the fact that it was the only treatment train that incorporated a biological filtration process (GAC) in which a portion of the biodegradable DOC was already removed. Escobar & Randall (2001) showed that although NF removed up to 97% of the BDOC, no significant AOC was removed and that aligned with the findings of this study. Despite the high level of DOC removal through the MF/NF treatment stream, subsequent loss through the PDS was comparable to other streams (in percentage terms).
In a distribution system with a deliberate lack of disinfectant residual, conditions appeared to favour the proliferation of culturable organisms with considerably higher cell counts in samples at the outlet of the PDS.
Despite different bacterial cell counts entering each PDS from the four treatment streams, equivalent outlet cell numbers were achieved in new systems after 8 months' operation. Superior treatment streams took longer to reach equilibrium due to lower initial input counts of active bacteria and less biodegradable organic matter.
In the first year of operation, active bacterial numbers from the conventional coagulation stream (Conv) were significantly different to the other three treatment streams (MIEX/Coag; MIEX/Coag/GAC and MF/NF) but achieved a higher degree of similarity within the second year as biological stability established in all PDS.
Incorporation of a biological treatment reduced the overall DOC loss through the PDS by reducing BDOC within the treatment prior to distribution.
Overall, the data suggests that although improving treatment to produce better water quality delayed bacterial deterioration of drinking water through the PDS, maintenance of disinfection residuals would still be imperative to manage long term bacterial water quality.
The authors would particularly like to acknowledge the assistance of the Mt. Pleasant WTP and collaborators, Mr James Morran and Mr Paul Colby. This project was supported by Water Research Australia Limited, South Australian Water Corporation, United Water International, Grampians Wimmera Mallee Water, Western Australian Water Corporation, Delft University of Technology, DCM Process Control and Orica Watercare.