Critical barriers to safe and secure drinking water may include sources (e.g. groundwater contamination), treatments (e.g. treatment plants not properly operating) and/or contamination within the distribution system (infrastructure not properly maintained). The performance assessment of these systems, based on monitoring, process parameter control and experimental tests, is a viable tool for the process optimization and water quality control. The aim of this study was to define a procedure for evaluating the performance of full-scale drinking water treatment plants (DWTPs) and for defining optimal solutions for plant upgrading in order to optimize operation. The protocol is composed of four main phases (routine and intensive monitoring programmes – Phases 1 and 2; experimental studies – Phase 3; plant upgrade and optimization – Phase 4). The protocol suggested in this study was tested in a full-scale DWTP placed in the North of Italy (Mortara, Pavia). The results outline some critical aspects of the plant operation and permit the identification of feasible solutions for the DWTP upgrading in order to optimize water treatment operation.

INTRODUCTION

In the last few years, drinking water treatments have become more and more complex due to the deterioration of supply sources and the implementation of more restrictive legislation limits. For example, in Europe, the Drinking Water Directive 98/83/EC (European Commission Council Directive 98/83/EC of 3 November 1998) (in Italy accomplished by the Legislative Decree 31/2001) (Legislative Decree February 2nd, 2001), whose limits were based on the World Health Organization's guidelines for drinking water quality (WHO 1996), reduced the limit of As(tot) from 50 to 10 μg L−1. Furthermore, many drinking water treatment plants (DWTPs) have problems linked to technical and operation aspects, especially for small to medium-sized plants. To guarantee high and safe water quality and optimize DWTPs' operation, a methodological approach that includes performance assessment should be recommended as a viable protocol. In literature, some authors developed different methods for evaluating the operation of DWTPs (Chang et al. 2007; Vieira et al. 2008; Zhang et al. 2012; Lamrini et al. 2013). Furthermore, WHO guidelines (WHO 2004, 2011) introduced a risk-based approach, called water safety plan (WSP), which is aimed to perform a system assessment, from catchment to consumer, and ensure safe drinking water. Since the DWTP functionality plays a fundamental role in the drinking water supply system in providing high-water quality, the adoption of a procedure for the DWTPs' performance assessment and optimization, which can be incorporated in a WSP or work in synergy with it, could be very useful.

In this context, the objective of this research was to propose a protocol for assessing DWTPs' operation and optimizing their efficiency in terms of water quality control and contaminant removal. This protocol was applied to assess the functionality of a full-scale DWTP located in Mortara (Pavia, Italy).

MATERIALS AND METHODS

Methodological approach

The assessment procedure, schematized in Figure 1, is composed of four phases:
  • Phase 1: Routine monitoring programme (RMP) – Routine monitoring is carried out and the historical data concerning DWTP operation are collected, in order to assess the effectiveness of the treatment processes (e.g. raw water characteristics, compliance with normative limits, energy consumptions, etc.).

  • Phase 2: Intensive monitoring programme (IMP) – If the results of the ‘RMP’ do not meet the requirement of the operator, an intensive monitoring programme is applied to evaluate each process of the DWTP (e.g. geometrical data, operative parameters, etc.). Moreover, some experimental laboratory tests (e.g. oxygen uptake rate tests, filtration capacity tests, etc.) are carried out in order to estimate the efficiency of each treatment process. In this way the operator should be able to identify DWTP problems.

  • Phase 3: Experimental studies (ES) – If necessary, alternative scenarios are evaluated through experimental tests at the laboratory and/or at pilot scale (e.g. column filtration test, oxidant demand, etc.). The results of these tests are useful in defining the optimal solutions for the DWTP upgrading, both as structural (e.g. new processes, new filter materials, etc.) and operational (e.g. filter media backwashing optimization, optimization of the operational parameters, etc.) interventions.

  • Phase 4: Upgrading and optimization (UO) – One or more upgrading interventions, defined in the previous phase, are implemented in the full-scale DWTP and then monitored in order to optimize their operational conditions. At this point the RMP (‘Phase 1’) is repeated in order to evaluate and verify the improvement of plant performance.

Figure 1

Scheme of the methodological approach proposed in order to assess DWTPs' performance and to support their optimization.

Figure 1

Scheme of the methodological approach proposed in order to assess DWTPs' performance and to support their optimization.

Description of the drinking water treatment system

The DWTP was built in 2001 and treats a maximum flow of 140 m3 h−1. In Figure 2, the sequence of treatments and the operational parameters are reported. The plant is fed with water drawn from a 200-m depth aquifer and the main contaminants in raw water are ammonium (0.79 mg L−1), Mn (79 μg L−1) and As(tot) (12 μg L−1). A pre-oxidation with air is used to oxidize iron (Fe) and form the precipitates which can be removed by sand filtration; the biofiltration removes through biological nitrification and a second oxidation with ozone (O3) allows completion of the oxidation of manganese (Mn) that also partially occurs in the biofiltration. Then, insoluble manganese oxides and organic micropollutants are subsequently removed by granular activated carbon (GAC) filters. Finally, an in-line disinfection with chlorine dioxide (ClO2) is applied before water distribution.

Figure 2

The DWTP of Mortara (Pavia, Italy). EBCT: empty bed contact time; HRT: hydraulic retention time; HSL: hydraulic surface load; WSS: water supply.

Figure 2

The DWTP of Mortara (Pavia, Italy). EBCT: empty bed contact time; HRT: hydraulic retention time; HSL: hydraulic surface load; WSS: water supply.

Experimental methods

Phase 1: Routine monitoring programme (RMP)

In this phase, the historical data on raw and treated water quality of 5 years (from 2005 to 2010), collected once a month, were analysed and assessed.

Phase 2: Intensive monitoring programme (IMP)

A 6-months IMP was carried out. Samples were collected twice a week, before and after each treatment, and the following chemical/physical parameters were analysed: , , , Fe, Mn, As(III), As(V), temperature, pH, dissolved oxygen (DO) and total suspended solids (TSS).

To assess the biological activity of the sand and GAC filters, oxygen uptake rate (OUR) and ammonium uptake rate (AUR) tests were performed on both backwash water (performed in endogenous conditions on 500 mL of backwash water which should contain the biomass detached from filter media) and samples of sand and GAC media directly collected from the top and the bottom of the filters (performed in exogenous condition re-suspending the biomass attached on 1 g media sample in 500 mL of pre-oxidized water). OUR tests were carried out according to Standard Methods (APHA et al. 2012) while AUR tests were performed following the procedure set out by Foladori et al. (2012).

Phase 3: Experimental studies (ES)

In this phase, experimental studies were carried out in order to evaluate some critical aspects highlighted in the two previous phases. In particular, the tests were focused on GAC filtration and treatment for arsenic oxidation/removal.

Since GAC filters did not properly operate, in order to choose the best one for the DWTP upgrading, eight different activated carbons (five of vegetable origin and three of mineral origin) were characterized by determining the Freundlich Isotherm according to ASTM D3860 – 98 (2008). Experimental data were elaborated to define activated carbon's adsorption and to identify the best carbon in terms of removal of organic matter (measured as UV absorbance at 254 nm) that was adopted as a target contaminant (with an initial UV absorbance at 254 nm of 0.0497 cm−1). These tests were carried out, for each type of activated carbon, with 500 mL of water sampled after the biofiltration process (14 hours after the last backwashing), with different carbon dosages (0, 1, 5, 10, 25 and 50 mg) and 1-hour contact time between water/GAC in stirring conditions (130 rpm).

Moreover, different experimental tests were performed to optimize arsenic removal through chemical oxidation and/or media adsorption.

The arsenic chemical oxidation with KMnO4 was evaluated by means of batch tests (Jar-test equipment) carried out on both raw and pre-oxidized (with air) water. These tests were performed in stirring conditions (130 rpm), with 5 minutes of contact time and KMnO4 dosages equal to 0.1–0.2–0.5 mg L−1 (dosages obtained in accordance with the standard methods for determination of the oxidant demand; APHA et al. 2012). Moreover, additional tests were performed on both raw and pre-oxidized water (with air) dosing 0.5 mg L−1 of NaMnO4.

The adsorption capacity of pyrolusite was investigated through batch tests (Jar-test equipment) on both raw and pre-oxidized water (with air). Further batch tests were carried out with pyrolusite and chemical dosage of KMnO4 (0.1 and 0.5 mg L−1). Each batch test was carried out in stirring conditions (130 rpm), with a contact time of 14.5 minutes (like the empty bed contact time (EBCT) of the biofiltration treatment of the real DWTP) and monitoring the As(tot), As(III) and As(V) concentration in the water sample before and after each test.

Additionally, pyrolusite was tested on a pilot scale plant operating in continuous condition with a 1 L h−1 upflow rate and an EBCT of 5 minutes. The monomedia filter (12 cm height) was reproduced in a glass column (3 cm diameter, 60 cm total height) fed with pre-oxidized water coming directly from the DWTP. As(tot), As(III) and As(V) were analysed daily on raw and treated water. Then, in order to prevent the influence of a photocatalytic oxidation of As(III), the pilot plant was covered.

After the analysis of the experimental data, four different scenarios of upgrading were defined and tested at laboratory-scale using Jar-test equipment. For each scenario, As(tot), As(III) and As(V) were measured in raw and treated water in order to identify the best sequence of processes in terms of arsenic removal yields.

Phase 4: Upgrading and optimization (UO)

In accordance with the plant operator, the best train of treatments in term of arsenic removal was implemented in the full-scale DWTP and an intensive monitoring programme was carried out in order to verify the plant performance.

Analytical methods

The sample collection, conservation and determination of the chemical/physical parameters (, , , Fe, Mn, As, temperature, pH, DO and TSS) were performed according to Standard Methods (APHA et al. 2012).

Arsenic speciation was obtained by means of filtration through an anionic resin (Serdolit® AS-1) capable of selectively removing As(V).

RESULTS AND DISCUSSION

Phase 1: Routine monitoring programme (RMP)

The RMP was carried out during the period 2005–2010 and the water quality compliance with the legislation limits (Legislative Decree 31/2001) was verified for the following critical parameters: (0.79 mg L−1), Mn (79 μg L−1) and As(tot) (12 μg L−1). The monitoring data show that ammonium and manganese were properly removed below the legislative limits (0.5 mg L−1 and 50 μg L−1, respectively), while arsenic was close to the maximum allowable concentration of 10 μg L−1 as no specific process for arsenic removal was implemented in the plant.

Phase 2: Intensive monitoring programme (IMP)

The IMP showed that ammonia was mainly removed by the GAC filter (60%), and not in the biofilter (20%) as expected, while manganese was primarily removed by the sand filter (70%) instead of the GAC filtration (15%). Moreover, arsenic was not removed in the treatment plant at all. In Figure 3, the contribution of each treatment to the overall removal of , Mn and Fe is reported.

Figure 3

Contribution of each treatment to the overall removal of the main contaminants.

Figure 3

Contribution of each treatment to the overall removal of the main contaminants.

The respirometric tests, carried out on backwash water and filter media, showed the presence of a biological activity on the GAC filters while it was almost absent in the biofiltration process, as in this stage negligible OUR and AUR values were obtained both in backwash water and on the filter media. As concerns the GAC filtration, the respirometric tests showed that the nitrifying biomass grows in the upper part (OUR = 4.4 mgO2 gTSS−1 h−1; AUR = 6.6 gTSS−1 h−1) and not in the lower part (OUR = 0.5 mgO2 gTSS−1 h−1; AUR = 0.6 gTSS−1 h−1) of the GAC filters. The different biological activity may be due to the decrease of biomass attached to the GAC granules with increasing filter depth (Kihn et al. 2000). Moreover, such a high difference in biological activity between the upper and the lower part of the filter may be due to an inadequate backwashing procedure that can inhibit a homogeneous biomass growth on the filter (Gibert et al. 2013).

Phase 3: Experimental studies (ES)

A series of tests was carried out in order to identify a new type of activated carbon for replacing the one already used in the plant. The results (Table 1) show that the vegetal carbons had higher removal yields of organic matter and porosity did not seem to influence their efficiency.

Table 1

Removal yields of organic matter evaluated in terms of UV 254 nm absorbance obtained for eight different types of GAC

Sample Carbon type Porosity Absorbance at 254 nm [cm−1η [%] 
V1 Vegetable Micro 0.0047 91 
V2 Vegetable Macro 0.0051 90 
V3 Vegetable Micro 0.0226 55 
V4 Vegetable Micro–meso–macro 0.0099 80 
V5 Vegetable Micro 0.0080 84 
M1 Mineral Macro 0.0001 100 
M2 Mineral Micro–meso 0.0089 82 
M3 Mineral Macro–micro 0.0277 44 
Sample Carbon type Porosity Absorbance at 254 nm [cm−1η [%] 
V1 Vegetable Micro 0.0047 91 
V2 Vegetable Macro 0.0051 90 
V3 Vegetable Micro 0.0226 55 
V4 Vegetable Micro–meso–macro 0.0099 80 
V5 Vegetable Micro 0.0080 84 
M1 Mineral Macro 0.0001 100 
M2 Mineral Micro–meso 0.0089 82 
M3 Mineral Macro–micro 0.0277 44 

Afterwards, experimental tests were carried out at laboratory scale in order to identify specific treatments able to optimize arsenic removal.

The results of the oxidation tests with KMnO4 and NaMnO4 on raw water (Figure 4) show that a good oxidation efficiency (>80%) can be achieved with KMnO4 after a contact time of 5 minutes and also with sub-stoichiometric dosages (<0.2 mg L−1). Moreover, the results of tests carried out with the same dosage of different chemical oxidant (0.5 mg L−1) have shown that NaMnO4 led to a lower yield of oxidation than the one obtained with KMnO4 (42% with respect to 83%). Furthermore, the dosage of the chemical oxidant in water preventively aerated enables higher yields of oxidation (>90%) with both KMnO4 and NaMnO4, as part of As(III) was oxidized to As(V) during the aeration.

Figure 4

Results of the oxidation batch tests with different chemical oxidants and dosages on both raw water (a) and water pre-oxidized with air (b).

Figure 4

Results of the oxidation batch tests with different chemical oxidants and dosages on both raw water (a) and water pre-oxidized with air (b).

The adsorption of arsenic using pyrolusite was investigated and good arsenic removal efficiency (>90%) was obtained on both raw and pre-oxidized water (with air). Moreover, the dosages of the chemical oxidant (KMnO4) did not significantly increase the yield of removal.

Then, a monomedia filter composed of pyrolusite was evaluated by means of a pilot plant fed with pre-oxidized water with an average concentration of 10 μg L−1 of As(tot) (about 70% As(III)). The experimental results (Figure 5) show that the media saturation was reached quickly (before 2,000 bed volumes) even if the oxidation capacity of the filter media was still effective, as shown by the fact that after 6,000 BV the outlet concentration of As(III) was still about 50% of the total As concentration.

Figure 5

Arsenic concentration and speciation in the treated water coming from the pilot plant (average concentration of As(tot) in inlet water equal to 10 μg L−1).

Figure 5

Arsenic concentration and speciation in the treated water coming from the pilot plant (average concentration of As(tot) in inlet water equal to 10 μg L−1).

Finally, according to the results of Phase 3, four different scenarios for upgrading were identified and tested at laboratory scale (Figure 6).

Figure 6

Four alternative schemes of water treatment tested at laboratory scale.

Figure 6

Four alternative schemes of water treatment tested at laboratory scale.

The As(tot) and As(III) concentrations in treated water, for each treatment train, are reported in Figure 6.

In the first scenario a pre-oxidation with air was followed by a biofiltration on pyrolusite (30%) and quartzite (70%) and a final in-line disinfection with ClO2. The results show poor removal yields of As(tot) (21%), although good oxidation of As(III) to As(V) is achieved. Since arsenic, especially if in As(V) state, can be effectively removed by coagulation/filtration (Hering et al. 1997), in the second scenario, an in-line dosage of 4 mg L−1 of FeCl3 (this dosage was defined through coagulation curve – data not shown – defined according to ASTM D2035 – 80 (2003)) was added between pre-oxidation with air and biofiltration. This configuration slightly improved the removal of As(tot) up to 36%.

In order to increase As(III) oxidation to As(V) and improve the removal of arsenic, in the third scenario a chemical oxidant (0.5 mg L−1 of KMnO4) was added just before FeCl3. In this case, a yield of removal of As(tot) equal to 57% was obtained.

In the fourth scenario the highest arsenic removal yield was reached (68%) by moving the dosages of both KMnO4 and FeCl3 after the biofiltration and adding a step of quartzite (50%)/GAC (50%) filtration in order to retain the chemical precipitates of the coagulation process.

Phase 4: Upgrading and optimization (UO)

The scenario chosen for the upgrading of the plant was the fourth one among the alternative schemes reported in Figure 6 as this process was able to optimize the As removal. The results of the intensive monitoring of the upgraded plant (data not reported) show that the removal of ammonia and manganese is carried out by the biofilter with yields of removal higher than 90% and the average concentration of As(tot) in treated water is equal to 7 μg L−1 (yield of removal equal to 40%).

CONCLUSIONS

The aim of this work was to define a protocol for the assessment of a full-scale DWTP based on monitoring, experimental tests and performance assessments. The validation of this protocol was done by applying a real-case study. The proposed protocol has shown to be useful for the identification of issues related to the DWTP process and planning of the operational and/or structural upgrading. Moreover, the modularity of the protocol makes it flexible and easy to apply to other DWTPs.

Focusing on treatment processes, this protocol could, also, be integrated in WSP as support for operational monitoring helping to evaluate corrective actions in order to improve the quality and minimize the risks of distributed water. This is a very interesting perspective as in the next revision of the European Drinking Water Directive a risk-based approach, similar to WSP, should be included (Hulsmann 2009).

In future work, economic aspects, such as energy consumption for water treatment, should be taken into consideration in order to further improve DWTPs' optimization.

According to the specific case study analysed in this work, the following conclusions can be reported.

  • Phase 1 (RMP) allowed a ‘state of the art’ of the plant, highlighting critical issues of the plant, such as the absence of specific treatments for arsenic removal.

  • Phase 2 (IMP) permitted identification of some critical issues in the operation of the DWTP, such as the incongruence of the project (the biological activity took place in the GAC filters instead of the sand filters) and the uselessness of the oxidation with ozone process (Mn removal was carried out by the biofiltration).

  • During Phase 3 (ES) different solutions for arsenic removal (oxidation and adsorption) were investigated by means of laboratory-scale and pilot-scale tests; the results identified four different scenarios of treatments for the plant upgrading which were reproduced at laboratory scale in order to study the removal of arsenic and its speciation.

  • Phase 4 (UO) was carried out after the upgrading of the DWTP, verifying the correct functionality and effectiveness of the new sequence of treatments.

The fourth scenario resulted in optimization of As removal and, therefore, it was implemented in the full-scale DWTP.

ACKNOWLEDGEMENTS

The authors would like to thank ASMortara Spa and ASMare srl, the company that manages the DWTP studied in this research. Special thanks to the company's technical staff who took part in this experimentation. Sabrina Sorlini and Cristina Collivignarelli planned and supervised the research activities and the paper drafting; Barbara Marianna Crotti did the data collection/analysis with Federico Castagnola, who also contributed to the paper draft; Massimo Raboni contributed to the experimentation.

REFERENCES

REFERENCES
American Public Health Association/American Water Works Association/Water Environment Federation
2012
Standard Methods for the Examination of Water and Wastewater
,
22nd edn
.
American Public Health Association, Washington, DC, USA
.
ASTM D2035 – 80
2003
Standard Practice for Coagulation-Flocculation Jar Test of Water
.
ASTM International
,
West Conshohocken, PA, USA
.
ASTM D3860 – 98
2008
Standard Practice for Determination of Adsorptive Capacity of Activated Carbon by Aqueous Phase Isotherm Technique
.
ASTM International
,
West Conshohocken, PA, USA
.
Chang
E.-E.
Chiang
P.-C.
Huang
S.-M.
Lin
Y.-L.
2007
Development and implementation of performance evaluation system for a water treatment plant: case study of Taipei water treatment plant
.
Practice of Hazard, Toxic, and Radioactive Waste Management
11
(
1
),
36
47
.
European Commission Council Directive 98/83/EC of 3 November 1998 on the quality of water intended for human consumption. Official Journal of the European Communities L 330 (1998)
32
54
.
Foladori
P.
Menapace
V.
Villa
R.
2012
Misura dei parametri cinetici del fango biologico mediante test respirometrici, titrimetrici, AUR, NUR e PUR (Measurement of the kinetic parameters of biological sludge through respirometric, titrimetric, AUR, NUR and PUR tests)
. In:
Impianti di Trattamento Acque: Verifiche di Funzionalità e Collaudo
(
Bertanza
G.
Collivignarelli
C.
, eds).
Hoepli
,
Milan, Italy
, pp.
265
312
.
Gibert
O.
Lefèvre
B.
Fernández
M.
Bernat
X.
Paraira
M.
Calderer
M.
Martínez-Lladó
X.
2013
Characterising biofilm development on granular activated carbon used for drinking water production
.
Water Research
47
(
3
),
1101
1110
.
Hering
J. G.
Chen
P.-Y.
Wilkie
J. A.
Elimelech
M.
1997
Arsenic removal from drinking water during coagulation
.
Journal of Environmental Engineering
123
(
8
),
800
807
.
Hulsmann
A.
2009
Decision time for Europe's Drinking Water Directive revision
.
Water21
11
(
1
),
42
44
.
Kihn
A.
Laurent
P.
Servais
P.
2000
Measurement of potential activity of fixed nitrifying bacteria in biological filters used in drinking water production
.
Journal of Industrial Microbiology and Biotechnology
24
(
3
),
161
166
.
Lamrini
B.
Lakhal
E. K.
Le Lann
M. V.
2013
A decision support tool for technical processes optimization in drinking water treatment
.
Desalination and Water Treatment
52
(
22–24
),
4079
4088
.
Legislative Decree February 2nd, 2001, No. 31. Attuazione della direttiva 98/83/CE relativa alla qualità delle acque destinate al consumo umano (Implementation of the Directive 98/83/EC on the quality water intended for human consumption). Gazzetta Ufficiale n. 52, 03/03/2001
.
Vieira
P.
Alegre
H.
Rosa
M. J.
Lucas
H.
2008
Drinking water treatment plant assessment through performance indicators
.
Water Science and Technology: Water Supply
8
(
3
),
245
253
.
World Health Organization (WHO)
1996
Guidelines for Drinking-Water Quality
,
2nd edn
.
World Health Organization
,
Geneva, Switzerland.
World Health Organization (WHO)
2004
Guidelines for Drinking-Water Quality
,
3rd edn
.
World Health Organization
,
Geneva, Switzerland
.
World Health Organization (WHO)
2011
Guidelines for Drinking-Water Quality
,
4th edn
.
World Health Organization
,
Geneva, Switzerland
.
Zhang
K.
Achari
G.
Sadiq
R.
Langford
C. H.
Dore
M. H. I.
2012
An integrated performance assessment framework for water treatment plants
.
Water Research
46
(
6
),
1673
1683
.