Abstract

A novel treatment was tested with groundwater to investigate its arsenic removal under natural conditions. The system utilised in-line electrochlorination to oxidise water constituents without the need for external chemical supply. The oxidised arsenic and iron co-precipitated and were filtered via Greensand Plus™. The filter was catalytically active and provided an emergency oxidant. The system had only a few maintenance requirements due to online water quality monitoring. The contaminant removal during the field test in Costa Rica was impaired by strong fluctuations in water quality including low iron concentrations. However, the system removed on average 68% of the arsenic. Mean values of arsenic were 40 ± 23 μg/L in groundwater and 13 ± 6 μg/L in treated water. Iron was removed from an average of 2.8 ± 2.4 mg/L to 0.2 ± 0.2 mg/L (93% removal). Free chlorine produced and available in the treated water tank had a mean concentration of 1.25 mg/L and 0.64 mg/L, respectively.

INTRODUCTION

Arsenic contamination is an issue of worldwide concern. A variety of health problems are associated with arsenic intake as it can affect the human skin, kidneys, the respiratory, nervous and cardiovascular system and can cause different forms of cancer (WHO 2004). Therefore the guidelines for drinking water quality published by the World Health Organisation (WHO 2008) advocate a maximum arsenic level of 10 μg/L. This value is also manifested in national regulations, e.g. in Costa Rica's regulation for drinking water quality (CMH 2015). Groundwater sources can naturally be contaminated by arsenic through its mobilisation from metalloid minerals like arsenopyrite, from which it is released as arsenous acid under reducing conditions. Millions of people are affected by arsenic-contaminated drinking water containing more than the WHO's recommended maximum level. As many of the people live in rural areas of developing countries, water provision is usually based on decentralised water supply.

Traditional options for arsenic removal include adsorption, coagulation/flocculation and membrane separation. Most arsenic treatment techniques rely on oxidation as a pre-treatment for enhanced removal efficiency and toxicity reduction as toxicity depends on the valence state (Ratnaike 2003). Arsenic in groundwater is mostly present in its trivalent state as arsenous acid (arsenite, H3AsO3). Methods for arsenic oxidation to its pentavalent form (arsenic acid, arsenate, H3AsO4) include solar, biological and chemical oxidation. Air oxidation requires several days to weeks for a complete oxidation of As(III). Pure oxygen reduces the required time to hours or days. Strong oxidising agents such as ozone, potassium permanganate or chlorine can further accelerate the process to a magnitude of minutes (Bissen & Frimmel 2003). Dodd et al. (2006) showed that 0.1 mg/L Cl2 dosed as free available chlorine (FAC = HOCl and OCl) into a solution of water spiked with 50 μg/L As(III) achieved an oxidation within 10 seconds. Electrochemical oxidation as electrochlorination produces the chlorine species hypochlorous acid (HClO) and is therefore a suitable option.

Residual chlorine in the treated water is needed to maintain a microbially safe drinking water. The necessary residual chlorine concentration for community water supply is pH dependent and in the range of 0.2–0.6 mg/L (WHO 2008).

Arsenic adsorbs better when freshly (co-)precipitated with iron (hydr)oxide flocs in comparison with adsorption onto older iron (hydr)oxides, as proven by Cao et al. (2008). The authors have investigated arsenic removal from artificial groundwater during iron removal via chemical oxidation using chlorine and filtration. The authors suggest an evaluation of the effects of real groundwater in pilot-scale studies.

The Solarex project approached this research gap to investigate the technical solution tailored to the specific needs in a rural setting. The aim was to refine the existing Sun Meets Water (SuMeWa) disinfection plant of the AUTARCON GmbH for the removal of arsenic. This system was designed to treat co-occurring iron and arsenic by means of anodic oxidation and remove them via adsorption and physical filtration utilising Greensand Plus™ (GSP). The extended SuMeWa system for decentralised drinking water production can supply up to 1,000 people per day.

This study presents the realisation, adaptation and scientific findings of the long-term pilot plant operation in Costa Rica.

METHODS

Technical background

The basic principle of in-line electrochlorination is employed – in particular the process of producing hypochlorite at mixed oxide electrodes utilising the natural chloride content of the groundwater. The in-line produced chemical agent (Equations (1)–(3)) can be utilised for disinfection and oxidation of iron, manganese and arsenic to enhance their removal. In the process dissolved bivalent iron is oxidised (Equation (4)) thereby forming iron (hydr)oxides having a high adsorption capacity for arsenic. Arsenic oxidation (Equation (5)) is followed by co-precipitation and adsorption. The treatment is finalised by a subsequent filtration step using GSP to hold back the precipitate. As a reactive medium with a manganese dioxide coating, GSP can temporarily compensate for low concentrations of hypochlorite production through the reduction of its reactive layer to oxidise the arsenic (Equation (6)). The (reduced) MnOx coating is continuously regenerated to MnO2, using the oxidants provided by the electrolytic cell.  
formula
(1)
 
formula
(2)
 
formula
(3)
 
formula
(4)
 
formula
(5)
 
formula
(6)

The modified treatment technology

The process design was modified from the SuMeWa disinfection plant for decentralised drinking water supply, normally incorporating a solar power panel. Pilot plant locations in Germany, India and Costa Rica have tested the modified system under different conditions. Figure 1 depicts the schematic setup of the system installed in Costa Rica including the sampling points (SP) for water quality evaluation.

Figure 1

Schematic of the pilot plant in San José.

Figure 1

Schematic of the pilot plant in San José.

The main components installed were a DC suction pump connected to a 1,100 L storage tank, feeding the electrolytic cell which was followed by a flow rate sensor. Dimensionally stable titanium electrodes coated with mixed oxides of the platinum group (MOX-electrodes), mainly iridium and ruthenium, were used in the setup. The electrodes had a surface area of 300 cm² and were operated using a current of up to 5 A depending on the need for oxidants. The centre piece for the automation of the system was the control unit incorporating a data recording and transfer device enabling online process monitoring. A filtration unit consisting of a 17.8 cm diameter pressure vessel filled up to a bed height of 65 cm with GSP was placed behind the flow meter. The filter was automatically backwashed in defined time intervals. The granular filter media GSP had a bulk density of 1,410 kg/m3, a porosity of 0.45 and an effective grain size between 0.3 and 0.35 mm (Inversand 2013). Having passed the filtration step, the treated water was collected in a freshwater storage tank equipped with two ORP sensors (JUMO tecLine Rd, Fulda, Germany) which transferred water quality data back to the control unit for process flow and electricity regulation. A site-specific target ORP value for the location was set based on the water matrix specific relationship between ORP and chlorine concentration sufficient for oxidation and disinfection. ORP measurements were used for water disinfection control as values above 650–750 mV reflect the antimicrobial potential of water irrespective of the water quality (Steininger 1985; Suslow 2004). The system operation started on 06/30/2015 with an initially high flow rate up to 426 L/h which was reduced to a constant rate of 100 L/h on 08/12/2016 followed by continuous operation.

Site description

The main requirements for site selection were a pH value between 6 and 8, a molar ratio for Fe/As of 10–20 assuring reliable co-precipitation and an inlet iron concentration of ≤5 mg/L to avoid incrustations in the system. The groundwater abstraction well located on the premises of the Instituto Costarricense de Acueductos y Alcantarillados (AyA) in la Uruca, San José, was considered suitable for testing as the water quality initially fit the requirements for pilot testing. The pump was located at 145.5 m depth of the 395 m deep well with an internal diameter of 35.5 cm. The pump extracted water at approximately 23 L/s. The well received water from several filter screens at five interval depths from 176 m to 390 m. The multi-layer aquifer consisted of layers of volcanic rocks made of lava and ignimbrite of high permeability alternating with low permeability layers formed by dense lava and clay. During long-term operation, constructions at the site's electric circuit led to irregularly occurring power cuts leading to system downtimes of up to multiple days. Coupled with pronounced fluctuations in the well's water quality, the plant had to endure challenging conditions.

Analytics and monitoring

Chemicals used were reagent grade. Solutions were prepared with reagent-grade water purified using an EASYpure 2 system (Thermo Fisher Scientific, Waltham, USA). Glassware cleaning and sampling protocols followed the recommendations of the standard methods (APHA, AWWA & WPCF 2005). One acidified sample with a 65% solution of HNO3 was taken to quantify the cations of iron and manganese as well as arsenic. A second untreated sample was taken to quantify chloride. Sampling was carried out to investigate the quality of raw groundwater (SP0), water stored in the feeding tank (SP1), water after the electrolytic cell (SP2) and the treated water that passed the filtration vessel (SP3) as shown in Figure 1. The monitoring period was from 06/30/2015 to 09/08/2016. In July–October 2015 and August–September 2016, sampling was intensified (twice per week) for a more detailed investigation. Free chlorine was measured on-site using a CHECKIT Direct MD200 photometer with a measuring range of 0.01–6.0 mg/L utilising DPD1 and DPD3 reagents (Tintometer GmbH, Division Aqualytic®, Dortmund, Germany). For the final sampling period in 2016, chlorine was measured using a HACH Pocket Colorimeter II at a wavelength of 528 nm with DPD powder packages in a measuring range between 0.1 and 8.0 mg/L (Hach Lange GmbH, Düsseldorf, Germany). Initially on-site electric conductivity (EC) was measured with a PCD 650 (OAKION, Vernon Hills, USA) and pH with a HI8424 pH meter (Hanna® instruments). From 08/12/2016 until 09/08/2016 the instant parameters (pH, EC, T) were determined using a Multi 3430 (WTW, Weilheim, Germany) combined with the sensors SenTix 940 (pH) and TetraCon® 925 (EC, T). Concentrations of arsenic were analysed in the laboratory by hydride generation atomic absorption spectroscopy method 3114 B, iron and manganese using Extraction/Air-Acetylene Flame Method 3111 C and silicon using method 3111 D (Direct Nitrous Oxide-Acetylene Flame Method). All methods were standard methods (APHA, AWWA & WPCF 2005) using an AAnalyst 800 (Perkin Elmer, Waltham, USA) at the TEC (Instituto Tecnológico de Costa Rica). The limits of quantification (LOQ) were 0.03, 4, 9 and 9 μg/L for arsenic, iron, manganese and silicon respectively. During sampling in 2015 iron was determined with the 1,10 phenanthroline method using the test kit LaMotte Smart2 combined with the photometer DR 900 (HACH). Chloride was measured using an ion-exchange chromatograph ICS 900 (Dionex, Sunnyvale, USA) at the HTWD (University of Applied Sciences Dresden, Germany) with a LOQ of 2 μg/L. Turbidity was analysed on-site using the device 2100Q (HACH) in a range of 0–1,000 NTU. Colour was determined using the portable photometer DR 900 (HACH) in a range of 15–500 Pt-Co.

RESULTS AND DISCUSSION

To evaluate the effectiveness of electrochlorination and filtration as a drinking water treatment technique, quality parameters are related to SP1, because it is the feeding water supplying the electrolytic cell. Precipitation of iron from the entering groundwater (SP0) in the storage tank (SP1) was noticed. Additionally, substantial changes in the well water quality were observed as pumping proceeded, resulting in high standard deviations. These changes may be linked to the extraction of water from different aquifer layers due to the continuous, high extraction rate from the well.

Pilot plant operation and modification

As the high inflow water quality fluctuations required flow adaptations to ensure longer filter residence times for arsenic removal, a new control unit implemented on 08/10/2016 enabled the manual assignment of a pre-set flow setting between 100 and 250 L/h. The flow was fixed to 100 L/h from 08/12/2016.

Chlorine production

Figure 2 shows the concentration of chloride available for chlorine production in the stored groundwater (SP1) and the resulting ORP at SP3. The mean concentration and its standard deviation present were very high at 635 ± 197 mg/L (n = 21). The concentration generally decreased during long, continuous pumping periods but never reached low levels that were of concern for sufficient chlorine production.

Figure 2

Chloride concentrations in feed water (n = 21) and ORP in treated water (n = 31).

Figure 2

Chloride concentrations in feed water (n = 21) and ORP in treated water (n = 31).

On 08/10/2015 the residual free chlorine (Figure 3) in the treated water was low, indicating possible additional chlorine consumption caused by the regeneration of the GSP medium's MnOx-coating in the filter unit. This underlines the beneficial effect of the GSP's oxidising capability for interruptions of chlorine production, which can be caused by very low chloride feed concentrations or power cuts. During the following intense testing period in October 2015 the total residual chlorine concentrations were in the desired range for microbiologically safe water storage and below values that impair the water's taste and odour. The goal to produce sufficient agents to oxidise water constituents while maintaining sufficient residual disinfectant was constantly reached after 08/10/2016 even though more power cuts occurred.

Figure 3

Free chlorine at the output of the electrolytic cell and in the treated water (n = 26).

Figure 3

Free chlorine at the output of the electrolytic cell and in the treated water (n = 26).

The free chlorine produced by the electrolytic cell and available in the treated water tank (Figure 3) had a mean concentration of 1.25 ± 0.62 mg/L (n = 26) and 0.64 ± 0.45 mg/L (n = 26), respectively. Thus the residual chlorine concentration mostly matched the WHO's given range for community water supply of 0.2–0.6 mg/L. The residual chlorine largely defines the ORP value of the treated water (Figure 2). In the final intense testing period in September 2016 the values were steadily above 650 mV, which is considered to be in the range of conditions where germs cannot survive.

Removal of arsenic, iron and manganese

The effect of air oxidation was observed in the system when the well water (SP0) was stored in the feeding tank (SP1). Iron sludge, arsenic and manganese accumulated in the tank. Therefore, Table 1 summarises the mean concentrations of arsenic, iron and manganese in the stored groundwater (SP1) in comparison with the treated water (SP3). After the final system modification regarding filter velocity had been carried out on 08/12/2016, the residence time of the water in the filter was increased from about 3 min to 8 min. This change ensured a better adsorption and removal of the iron flocs with the adsorbed arsenic. Arsenic was removed from the groundwater to a mean concentration of 13 μg/L which was close to the local drinking water treatment requirement of 10 μg/L. Iron was removed to a mean concentration of 0.2 mg/L in the treated water, thereby meeting the water quality regulations. The manganese concentration of 119 μg/L in the stored groundwater was lowered to 68 μg/L.

Table 1

Arsenic, iron and manganese guidelines, concentrations and removal 06/30/2015–09/08/2016

Parameter Unit n Stored groundwater (SP1) Treated water (SP3) Removal in % Costa Rican guideline (CMH 2015WHO guideline (WHO 2011
Astot μg/L 25 40.1a ± 23.0b 13.0 ± 5.6 68 10 10 
Fetot mg/L 28 2.8 ± 2.4 0.2 ± 0.2 93 0.3* 0.2 
Mntot μg/L 19 119 ± 110 68 ± 84 43 500* 50 
Parameter Unit n Stored groundwater (SP1) Treated water (SP3) Removal in % Costa Rican guideline (CMH 2015WHO guideline (WHO 2011
Astot μg/L 25 40.1a ± 23.0b 13.0 ± 5.6 68 10 10 
Fetot mg/L 28 2.8 ± 2.4 0.2 ± 0.2 93 0.3* 0.2 
Mntot μg/L 19 119 ± 110 68 ± 84 43 500* 50 

*In well water containing Fe & Mn, the maximum combined concentration must be <300 μg/L.

aAverage concentration.

bStandard deviation.

Figure 4 shows the total arsenic concentrations in the feed water and the treated water together with the WHO guideline to be achieved. The arsenic concentrations of the feed water varied greatly and temporarily fell below concentrations of concern.

Figure 4

Total arsenic in feed water (n = 25) and treated water (n = 24).

Figure 4

Total arsenic in feed water (n = 25) and treated water (n = 24).

In agreement with laboratory testing done at the HTWD and results summarised in Twidwell et al. (2005), arsenic removal was challenging for the removal of already low arsenic inflow concentrations as they do not favour co-precipitation kinetics. This phenomenon was confirmed by the good correlation of the arsenic feed concentration to its removal in the field test (R = 0.73).

On 03/03/2016 a filter breakthrough of iron occurred, causing arsenic concentrations to peak as well. The maximum arsenic concentration in the treated water was 31.5 μg/L during this breakthrough. After the backwash setting had been adjusted to a more frequent (daily) interval and the residence times in the filter had been increased such events were precluded. Mahmood et al. (2014) also underline the importance of contact time and a sufficient concentration gradient (e.g. higher arsenic initial concentrations) between the solution and the surface area for arsenate adsorption processes. Another factor possibly impairing arsenic removal was the co-occurrence of competing ions as silicate, which had a concentration of 25 ± 1 mg Si/L (n = 2) in two samples taken one week apart in November 2016. A concentration of 14–23 mg/L Si was found to cause a 2.5–3.5 times higher demand of iron-based coagulants for arsenic removal compared to silicate-free systems during laboratory testing (Laky & Licskó 2011). Phosphate was not detected.

Iron was present in the stored groundwater at concentrations of 2.8 ± 2.4 mg/L (n = 28) and was decreased to residual concentrations of 0.2 ± 0.2 mg/L (n = 26) in the treated water (Figure 5). The maximum iron concentration at SP1 was 15.2 mg/L on 10/15/2015. It was accounted for by a breach of the iron holdback within the filter due to insufficient backwashing, resulting in elevated iron concentrations of 1.24 mg/L at SP3. The increase of the residence time had a slightly positive effect, achieving 2% more removal (−99.9% Fe) after the flow had been set to a constant rate of 100 L/h.

Figure 5

Total iron in feed water (n = 28) and treated water (n = 26).

Figure 5

Total iron in feed water (n = 28) and treated water (n = 26).

The pilot test demonstrated the dependence of arsenic removal on the filter velocity as well as on the iron and arsenic inflow concentrations. Phenrat et al. (2008) studied arsenic–iron hydroxide sludge finding that there is no formation of crystalline species like Fe(AsO)4 and that adsorption, precipitation, co-precipitation, and occlusion are the main immobilisation mechanisms of arsenic.

An increase in filter residence time by 167% from 3 minutes (phase 1) to 8 minutes (phase 2, after 08/12/2016) resulted in stable arsenic removal. This was an improvement as the inflow water quality was much less favourable for arsenic removal during phase 2 because (i) the mean iron inlet concentration was 39% lower (2.1 mg/L) and (ii) the arsenic inlet concentration was also lower by 7.5% (mean concentration of 35 μg/L) during phase 2.

Manganese occurred at concentrations of 119 ± 110 μg/L (n = 19) at SP1 and was removed to 68 ± 84 μg/L (SP3, n = 17). The manganese concentrations increased in both feed and treated water during the pilot operation.

Changes in other parameters

The mean pH values at SP1 and SP3 were very similar at 6.5 ± 0.2 (n = 31) and 6.6 ± 0.2 (n = 32) respectively. This pH is in the range for iron(III) hydr(oxide) formation (being the prerequisite for arsenic adsorption) from dissolved ferric iron at the ORP levels achieved by the plant. For As(V) adsorption itself, the pH is well suited as well.

Temperatures in the stored water tank were at a mean of 27 ± 2 °C (n = 23). There was no significant change in temperature due to the treatment.

Turbidity was reduced by 99.6% from a mean of 39 ± 1 NTU to 1 ± 1 NTU (n = 10) and colour was reduced by 87.6% from 124 ± 59 to 10 ± 5 Pt-Co (n = 9). Both parameters were in agreement with local regulations (CHM 2015) and WHO recommendations (WHO 2011).

One approach to handle the dehydrated backwash sludge is the embedding in bricks or cement for use in construction or for disposal in landfills. Lakshmanan et al. (2008) reported that Fe-As sludge easily passes the Toxicity Characteristic Leaching Procedure (TCLP) from the EPA (1992).

CONCLUSIONS

A compact disinfection system consisting of in-line electrolysis coupled with a filtration unit utilising Greensand Plus™ for arsenic removal during iron removal was tested. It required little maintenance due to online water quality and process monitoring. It achieved a good decolouration and disinfection of the water. The process was independent from chemical additions as the multi-purpose reagent was produced from naturally occurring chloride content thereby needing no storage or external addition of oxidants. A primary disinfection as well as a secondary disinfectant provision were ensured and the in-line produced reagent was also available for utilisation as an MnOx media regenerant.

The method was successfully applied for the simultaneous removal of iron and arsenic. Iron concentrations were lowered by 93% and arsenic concentrations by 70% but did not always achieve the removal goal of 10 μg/L arsenic. Contaminant removal in Costa Rica was impaired by various challenges like power cuts, an initially insufficient filter residence time and temporary low inflow concentrations of iron and arsenic, adversely affecting removal kinetics. Strong, sudden variations of the iron concentration in the raw water needed shorter filter backwash intervals for preventing filter breakthroughs during unexpectedly high iron concentrations. Peaks in arsenic concentrations in the treated water were mainly linked to insufficient iron removal as they occurred simultaneously with filter breakthroughs of iron which were later avoided with an automated filter backwash interval of 1 day. Possible other impairments were co-occurring species as silica.

The system was able to treat at least 2,400 L/d when continuous power supply was available. The amount of water treated could supply a small community with disinfected water. Pilot testing in Costa Rica demonstrated the importance of a sufficient filter velocity, as the increase of filter residence time resulted in stable arsenic removal even when there was a 39% lower iron inlet concentration available in the second phase of the testing period. More detailed research into filter velocities or additional filter settings are needed to adapt residence times and achieve further improvement of co-precipitation/adsorption/oxidation/filtration processes aiming at permanently low outflow concentrations of arsenic complying with the international standards. Business models for this decentralised plant are adaptable as the treatment process can be tailored to fit the local needs by modular extensions added to the technology.

ACKNOWLEDGEMENTS

This research was funded by the German Ministry of Education and Research (BMBF), KMU-innovative programme, project no. 02WQ1333A and B and supported by the Research and Extension Council of ITCR, Costa Rica, as well as the Heinrich-Stockmeyer-Foundation. Additional support was provided by staff and students of the Division of Water Sciences of the HTWD and the TEC. Further support was provided by staff of the Instituto Costarricense de Acueductos y Alcantarillados, in particular Rafael Orozco as well as the staff of the Centro de Investigación en Protección Ambiental of the TEC in Cartago.

REFERENCES

REFERENCES
APHA, AWWA & WPCF
2005
Standard Methods for the Examination of Water and Wastewater
.
21st edn
.
American Public Health Association
,
Washington, DC, USA
.
Bissen
,
M.
&
Frimmel
,
F. H.
2003
Arsenic – a review. Part II: oxidation of arsenic and its removal in water treatment
.
Acta Hydrochimica et Hydrobiologica
31
(
2
),
97
107
.
Cao
,
T. H.
,
Vu
,
N. D.
,
Vo
,
T. T. T.
&
Truong
,
T. M.
2008
Arsenic Removal from Water by Chemical Oxidation and Adsorption on In-situ Formed Ferric Hydroxide
. In:
Annual Report of FY 2007
,
the Core University Program between Japan Society for the Promotion of Science (JSPS) and Vietnamese Academy of Science and Technology (VAST)
, pp.
278
285
.
CMH
2015
Executive Decree No. 38924-S. Costa Rican Ministry of Health. 01.01.2015, Official Gacette. 09.01.2015
, pp.
1
34
.
Dodd
,
M. C.
,
Vu
,
N. D.
,
Ammann
,
A.
,
Le
,
V. C.
,
Kissner
,
R.
,
Pham
,
H. V.
,
Cao
,
T. H.
,
Berg
,
M.
&
Von Gunten
,
U.
2006
Kinetics and mechanistic aspects of As(III) oxidation by aqueous chlorine, chloramines, and ozone: relevance to drinking water treatment
.
Environ. Sci. Technol.
40
(
10
),
3285
3292
.
EPA
1992
Method 1311. Toxicity Characteristic Leaching Procedure
.
United States Environmental Protection Agency
,
USA
.
Lakshmanan
,
D.
,
Clifford
,
D. A.
&
Samanta
,
G.
2008
Arsenic Coagulation with Iron, Aluminum, Titanium, and Zirconium Salts
.
WERC and AWWA Research Foundation
,
USA
.
Mahmood
,
T.
,
Din
,
S. U.
,
Naeem
,
A.
,
Tasleem
,
S.
,
Alum
,
A.
&
Mustafa
,
S.
2014
Kinetics, equilibrium and thermodynamics studies of arsenate adsorption from aqueous solutions onto iron hydroxide
.
Journal of Industrial and Engineering Chemistry
20
(
5
),
3234
3242
.
Phenrat
,
T.
,
Marhaba
,
T. F.
&
Rachakornkij
,
M.
2008
Leaching behaviors of arsenic from arsenic-iron hydroxide sludge during TCLP
.
Journal of Environmental Engineering
134
(
8
),
671
682
.
Ratnaike
,
R. N.
2003
Acute and chronic arsenic toxicity
.
Postgraduate Medical Journal
79
(
933
),
391
396
.
Steininger
,
J. M.
1985
PPM or ORP: Which Should Be Used? Swimming Pool Age & Spa Merchandiser (November 1985)
.
Suslow
,
T. V.
2004
Oxidation–Reduction Potential (ORP) for Water Disinfection Monitoring, Control, and Documentation
.
Publication 8149
,
University of California
,
Davis
,
Department of Vegetable Crops, Oakland, CA, USA. Available at: http://anrcatalog.ucanr.edu/pdf/8149.pdf (accessed 15 November 2016)
.
Twidwell
,
L. G.
,
Robins
,
R. G.
&
Hohn
,
J. W.
2005
The Removal of Arsenic from Aqueous Solution by Coprecipitation with Iron(III)
. In:
Arsenic Metallurgy: Fundamentals and Applications
.
The Minerals, Metals & Materials Society
,
San Francisco, CA, USA
.
WHO
2004
IARC Monographs on the Evaluation of Carcinogenic Risks to Humans Volume 84: Some Drinking-water Disinfectants and Contaminants, Including Arsenic
.
IARC Press
,
Lyon, France
.
WHO
2008
Guidelines for Drinking-Water Quality
,
3rd edn
.
Incorporating first addenda.
Vol. 1
, Recommendations
,
WHO Press
,
Geneva, Switzerland
.
WHO
2011
Guidelines for Drinking Water Quality
,
4th edn
.
World Health Organization
,
Geneva, Switzerland
.