Environ Monit Assess (2015) 187:131 DOI 10.1007/s10661-015-4350-8
A continuous active monitoring approach to identify cross-connections between potable water and effluent distribution systems E. Friedler & Y. Alfiya & A. Shaviv & Y. Gilboa & Y. Harussi & O. Raize
Received: 19 August 2014 / Accepted: 9 February 2015 # Springer International Publishing Switzerland 2015
Abstract A continuous active monitoring approach was developed for identification of cross-connections between potable water supply systems and treated wastewater effluent reuse distribution systems. The approach is based on monitoring the oxidation reduction potential (ORP) at the potable water system while injecting sulfite (a reducing agent) into the effluent line. A sharp decrease in the ORP of the potable water would indicate a cross-connection event. The approach was tested in batch experiments on treated municipal wastewater effluent of varying degree of treatment, and at dilution ratios of up to 1:100 (effluent/ potable). The approach was then examined under continuous flow conditions, which simulated cross-connection events at various dilution ratios (up to 1:100). In the continuous runs, differences between the potable water ORP and the effluent–potable water mixture (containing sulfite as sodium bisulfite (SBS)) ORP were 450–630 mV. This suggests high potential for identifying a crossconnection event. Implementation of the approach includes adding sulfite to effluent used for agricultural irrigation; hence, possible effects on soil and on crops were studied in soil columns and pots planted with basil (Ocimum basilicum) as a model plant. No negative effects of sulfite addition to the irrigation effluent were observed E. Friedler (*) : Y. Alfiya : A. Shaviv : Y. Gilboa Faculty of Civil and Environmental Engineering and the Grand Water Research Institute, Technion–Israel Institute of Technology, Haifa, Israel e-mail: [email protected] Y. Harussi : O. Raize ADAN Technical and Economic Services Ltd., Tel-Aviv, Israel
in the irrigated soils and plants, and therefore, it could be safely implemented also in agricultural applications. Keywords Basil . Cross-connection detection . Dual distribution systems . Online monitoring . ORP . Public health . Plants; s . Soil . Sulfite . Wastewater reuse
Introduction Rapid population growth coupled with ever-increasing water demand for agricultural, municipal and industrial uses has led to depletion of natural water sources and water scarcity in many regions (Henderson et al. 2009; Hambly et al. 2010; Adewumi et al. 2010). Reuse of municipal wastewater effluent can alleviate this stress on natural water sources. Recycled wastewater is being used worldwide for many purposes, including agricultural irrigation, municipal non-potable uses (such as garden and park irrigation and toilet flushing), various industrial uses and even augmentation of potable water supplies (Hambly et al. 2012). As expected, waterscarce countries and regions are leading the reuse of recycled water; for example, over 80 % of the municipal wastewater effluent in Israel is reused predominantly for agricultural irrigation; in Australia, 19 % is being reused mostly for agricultural irrigation but also for urban nonpotable uses; in Spain, 11 % is being reused mostly for agricultural irrigation and for recreational and urban uses; California (southern) reuses 13 % of its treated wastewater, 37 % of which is reused for agricultural irrigation, 17 % for landscape irrigation and 12 % for
131
Page 2 of 18
aquifer recharge; in Singapore, highly treated effluent (termed NEWater) is reused for potable supply consisting 30 % of the national water supply (Newton et al. 2010; Vanham 2011; Cohen et al. 2012; Sato et al. 2013). With the increasing reuse of treated wastewater effluent, growing emphasis is being placed on minimising public health risks, part of which is related to the distribution network of the treated effluent. Dual distribution (or separate dual-pipe) system (one for potable water and one for treated wastewater effluent) is a key in the redistribution of recycled water for reuse. Dual distribution systems must be carefully managed to protect consumers’ safety from incorrect/incidental crossconnections between potable and recycled water lines, an event which has potential detrimental public health implications. Indeed, countries derived regulations tackling this issue. The major part of these regulations relate to infrastructure (e.g. purple colour of pipes conveying reclaimed effluent, clear signs marking reclaimed water distribution systems, one-way valves, and air gaps) or restricting the use of reclaimed water in certain areas (e.g. Israel Ministry of Health (2002) requires minimum distance between agricultural plots irrigated with treated effluent and residential/public buildings and roads). Notwithstanding, in large dual distribution systems, it is impossible to ascertain that cross-connections will never occur. Indeed, hundreds of incidents of cross-connections have been reported worldwide, mainly from the USA, Europe and Australia (Oesterholt et al. 2007; Mena et al. 2008; Henderson et al. 2009; Hambly et al. 2012; Pitkanen 2013). It is estimated that many more occurred but not reported in the scientific literature. Considering the fact that cross-connections are de facto inevitable to a certain (low) extent and the consequential negative health effects, there is a need to reliably detect cross-connections online, using an indicator that can consistently distinguish between potable water and treated effluent. Despite this need, only few studies were published on the subject. These tried to differentiate between potable and recycled wastewater by measuring parameters such as electrical conductivity (EC), total organic carbon (TOC), turbidity and UV-Vis absorption (Hambly et al. 2012). Hall et al. (2007) investigated how changes in water quality parameters, which potentially indicate contamination resulting from a cross-connection event, can be detected by real- or near-real-time sensors. They revealed that sensors that responded to most contaminants were those that monitored free chlorine, TOC, oxidation reduction potential (ORP), EC and chlorides. They stated that these
Environ Monit Assess (2015) 187:131
results may be used in a warning system for contamination within a water distribution system. Henderson et al. (2009) suggested using fluorescence spectroscopy as a potential monitoring tool, basing their suggestion on publications that reported that sewage intrusion to rivers and estuaries can be identified by fluorescence spectroscopy. However, they stated that in recycled wastewater systems where the effluent is highly treated, the impact of advanced treatment processes on this monitoring tool is largely unknown and requires investigation, and therefore, advanced data analysis techniques are required to improve the ability to detect contamination events. To improve the ability to monitor and rapidly identify (real-time) cross-connections between treated effluent and potable water distribution systems, we developed a continuous active monitoring approach. The approach is based on injection of a tracer into the effluent line and identification of its effects (not the tracer itself) in the potable water line as a signal of a cross-connection event. The chosen tracer is sodium bisulfite (SBS): when injected into the effluent line reduces its ORP that is usually lower than the ORP of potable water. Monitoring is performed by continuous measurement of the ORP in the potable water pipe where a significant decrease of its value would indicate a cross-connection event. Sodium bisulfite The term Bsulfiting agents^ is used to describe sulfur dioxide (SO2) and several forms of inorganic sulfite, which liberate sulfur dioxide under certain conditions (Nair et al. 2003). The sulfur system exists in equilibrium water solutions where the dominant species are pH dependent, for example, at pH 7.4 (37 °C), a mixture of sulfite ions (SO3−2) and bisulfite anions (HSO3−) predominates. More on sulfite reactions and kinetics can be found elsewhere (Andersen 2003). Sulfiting agents are used in the food industry, primarily to reduce or prevent spoilage and discoloration, to bleach food starches, to condition dough in baked foods, to control fermentation of wine and to soften corn kernels during wet milling (Nair et al. 2003). Concentrations above 100 ppm are found in dried fruits, while lower concentrations (10–100 ppm) are found at many other food products such as frozen and dried potatoes, pickles, gravies and jams. The World Health Organization (WHO) recommended acceptable daily intake level of 0.7 mg/kg body weight. The US Food and Drug Administration (USFDA) classified sulfiting agents as Bgenerally recognised as safe^
Environ Monit Assess (2015) 187:131
and as sources of vitamin B1 (thiamine). Their concentrations in wines and raw shrimp were respectively limited to 350 ppm (equivalent to 100 ppm SO2). There are some reports of adverse reactions by asthma patients following ingestion of sulfiting agents. Therefore, the USFDA requires packaged foods containing more than 10 ppm sulfite to list it on the ingredient label. Sulfiting agents are also used in the cosmetics industry as antioxidants. The USFDA does not require reporting their concentrations in cosmetics (Nair et al. 2003). Sodium bisulfite (SBS; NaHSO3), one of the sulfiting agents, is used by the cosmetics industry (Nair et al. 2003). In agriculture, it serves as a growth regulator used, at low concentrations, for enhancing photosynthesis to promote field crop yield (Wang et al. 2003; Guo et al. 2006; Yang et al. 2008; Bian et al. 2009). Guo et al. (2006) applied SBS to strawberry plants and reported a rise in net photosynthetic rate. Yang et al. (2008) reported that spraying low concentration of SBS on tea trees significantly enhanced the net photosynthetic rate and carboxylation efficiency. Bian et al. (2009) reported that SBS can accelerate transformation of chloroacetanilide herbicides to less toxic transformation products by dechlorination in aquatic environments. To summarise, there are reports in the literature on using SBS in agriculture to increase photosynthesis leading to increased yields, and there are some indications that it does not have a negative impact on crops. Nevertheless, only a scant number of reports were found and this issue deserves further investigation. This paper describes the development of the new monitoring method. It starts with the rationale of the method. Then, results of batch experiments where various effluent sources were mixed with potable water at different dilution ratios are presented. This is followed by results of a pilotscale continuous flow system where a cross-connection was purposely made at varying dilution ratios. Since the implementation of the approach includes addition of SBS to effluent used in many cases for agricultural irrigation, the last part of the paper experimentally examines its possible effects on soil and plants.
Materials and methods The proposed approach Following the literature review, we stated three requirements from a continuous real-time monitoring approach for identification of cross-connections: (1) The
Page 3 of 18 131
monitored parameter should be present in the recycled wastewater stream and should be easy to quantify (preferably online); (2) it would be beneficial if the presence of this parameter could be increased by a controlled manner, to produce a significant difference against its possible presence in potable water; and (3) as treated effluent is often used for irrigation, the chosen parameter should not have detrimental effects on the irrigation system, soil, crops or public health. In view of this, the chosen parameter should be soluble in water; be easily identified, low cost and available allowing continuous monitoring; should not damage the distribution system (e.g. clogging, sedimentation, fouling, corrosion); should not be toxic to humans, flora and fauna; and be approved for use in food industry. As aforementioned, the parameter chosen is oxidation reduction potential (ORP). ORP describes the ratio between oxidised and reduced chemical species in a solution, being high when a solution contains more oxidised species than reduced ones, and low vice versa. Potable water generally contains low concentrations of reducing compounds and usually some level of residual chlorine (for disinfection; oxidising species); therefore, its ORP values usually should be relatively high (from ~300 to ~900 mV depending on the source of water; Suslow 2004; Edzwald and Tobiason 2011). In contrast, the ORP of treated effluent that contains reduced species (e.g. organic substances) is usually lower. Even when the effluent is chlorinated, its ORP is expected to remain low due to the chlorine demand of the reducing substances present. During a cross-connection event, the two streams (treated effluent and potable water) mix and the ORP, measured at the potable water system, decreases. Yet, dilution of the two streams is expected. Hence, especially when the proportion of effluent in the cross-connected stream is lower than the proportion of potable water and its degree of treatment is high, the ORP decrease may be difficult to identify. To overcome this obstacle, SBS (a reducing substance) is added to the effluent in order to decrease its ORP and thus increase the difference between the ORP of the potable water and the effluent making the identification of a crossconnection easier. From an operational point of view, it should be sufficient to inject SBS to the effluent irrigation system immediately when irrigation starts for a certain period of time (e.g. 15–30 min). This should give a high enough signal that could be identified in the potable water distribution network should a crossconnection exist.
131
Page 4 of 18
Experimental methods The approach was developed and tested first by batch experiments in the laboratory, then by continuous pilotscale experiments where cross-connections were intentionally created. Both stages were performed with various dilution ratios between the effluent and the potable water streams. Possible impacts (positive or negative) of SBS on soil and plants were studied in soil columns and in planted-pots experiments. Batch experiments Batch experiments were performed with effluent from seven sources varying in the degree of treatment (Table 1), comprising of wastewater treatment plants (WWTPs) effluent (EF), effluent from effluent storage reservoirs (SRs) and reclaimed water (RW, i.e. Shafdan WWTP effluent after soil aquifer treatment (SAT) and some dilution with groundwater). To simulate a crossconnection event at a particular place, samples of potable water were taken from the vicinity of each effluent source. The potable water was chlorinated to meet one of the following concentrations: 0.1, 0.5 and 1.0 mg/L residual chlorine (after 30-min contact time). SBS was added to the effluent to achieve concentrations of 10, 20 and 30 mg SO−3/L. Nine combinations from each source were examined (three chlorine levels × three SBS levels), each at six dilution ratios: 1:1, 1:2, 1:10, 1:20, 1:50 and 1:100 (1:100 being one effluent volume in 100 potable water volumes). The ORP of the potable water (unchlorinated and chlorinated), effluent (with and without SBS) and effluent-water mixtures was measured with an ORP electrode (Sensorex S200C/BNC Comb pH Electrode on Eutech CyberScan 110 pH/ORP Portable Meter). All chemical analyses were performed according to the Standard Methods (Apha et al 2012). Continuous flow pilot-scale experiments A pilot plant was constructed at Shomrat SR, which stores the secondary effluent of Acre WWTP (Table 1). All pipes used were made of PVC; thus, no reaction between chlorine added and the pipe material was expected. The pilot system consisted of two separate streams, namely, potable water and treated effluent (Fig. 1); each was fed from a separate tank. Hypochlorite could be injected into the potable water line to obtain residual chlorine concentration of 0.1 up to
Environ Monit Assess (2015) 187:131
1 mg/L (after 30-min contact time). SBS could be dosed into the effluent line to reach its desired concentrations. The ORP of each stream was continuously measured by ORP electrodes (SRH-1, Seko Ltd). The two streams were intentionally cross-connected in a designated point where a third ORP electrode was placed. The pilot plant was operated through a computerised control system which controlled all valves, pumps, chlorine and SBS dosing, and dilution ratios between the two streams. All data (including flows, dilution ratios and readings of the three ORP electrodes) was stored in a data logger every 10 s. The mixing ratio of the two streams was controlled by proportional valves and bypass streams. Prior to each experiment run, the ORP electrode was checked and calibrated with reference solution (Redox Standard, 250 mV at 25 °C Ag/AgCl, Reagecon, Ireland). Precautions were taken to avoid health risk, including installation of non-return valve on the potable water stream and an air plug on the pipe feeding the potable water tank, and discharging the mixture (after the crossconnection point) to the SR. The system was operated in two modes: 1. Short experiments that tested the system’s ability to identify a cross-connection were examined at six dilution ratios (effluent/potable water): 1:2, 1:5, 1:10, 1:20, 1:50 and 1:100. The system was operated 6 h a day at three different dilution ratios (2 h each). During the first 30 min, SBS and chlorine were not added in order to monitor background ORPs of the three streams (effluent, potable water, mixture effluent-potable). During the next 60 min, SBS was dosed to the effluent stream and chlorine to the potable water one. Then, SBS and chlorine addition was halted and the system was let to stabilise in a different dilution ratio. During the whole period, ORP values were continuously monitored in all three streams. 2. Long experiments where the system was operated with the same dilution ratio for a long period (up to 3–4 weeks) and injection of SBS and chlorine were performed intermittently in order to simulate repeated irrigation with effluent.
Impact on soil and plants Possible effects of SBS addition on soil were studied in soil columns, and on basil (Ocimum basilicum) plants grown in pots. Basil was selected as a model plant due to
Environ Monit Assess (2015) 187:131
Page 5 of 18 131
Table 1 Characteristics of the effluent sources used in the batch experiments and in the pilot-scale system Experiment type
Batch
Pilot-scale (continuous)
Source
Ein Hashofet Nir Ezion Ra'anana Haifa
M. Kishon Shafdan
Type
SRa
SRb
–
EF
EF
EF
2.0°105
4.3°106
3.5°107 –
Shomrat
RW before SR RW after SR SRc –
Q
m3/y
V
3
m
2.3°10
pH
–
8.1
7.5
7.1
7.8
7.2
7.3
7.3
EC
mS/cm 3.9
0.89
1.2
1.7
0.97
0.83
1.4
TSS
mg/L
5.6
7.4
14
0.40
2.2
7.8
CODd
mg/L
15
80
12
~0
78
CODt
mg/L
69
A continuous active monitoring approach to identify cross-connections between potable water and effluent distribution systems E. Friedler & Y. Alfiya & A. Shaviv & Y. Gilboa & Y. Harussi & O. Raize
Received: 19 August 2014 / Accepted: 9 February 2015 # Springer International Publishing Switzerland 2015
Abstract A continuous active monitoring approach was developed for identification of cross-connections between potable water supply systems and treated wastewater effluent reuse distribution systems. The approach is based on monitoring the oxidation reduction potential (ORP) at the potable water system while injecting sulfite (a reducing agent) into the effluent line. A sharp decrease in the ORP of the potable water would indicate a cross-connection event. The approach was tested in batch experiments on treated municipal wastewater effluent of varying degree of treatment, and at dilution ratios of up to 1:100 (effluent/ potable). The approach was then examined under continuous flow conditions, which simulated cross-connection events at various dilution ratios (up to 1:100). In the continuous runs, differences between the potable water ORP and the effluent–potable water mixture (containing sulfite as sodium bisulfite (SBS)) ORP were 450–630 mV. This suggests high potential for identifying a crossconnection event. Implementation of the approach includes adding sulfite to effluent used for agricultural irrigation; hence, possible effects on soil and on crops were studied in soil columns and pots planted with basil (Ocimum basilicum) as a model plant. No negative effects of sulfite addition to the irrigation effluent were observed E. Friedler (*) : Y. Alfiya : A. Shaviv : Y. Gilboa Faculty of Civil and Environmental Engineering and the Grand Water Research Institute, Technion–Israel Institute of Technology, Haifa, Israel e-mail: [email protected] Y. Harussi : O. Raize ADAN Technical and Economic Services Ltd., Tel-Aviv, Israel
in the irrigated soils and plants, and therefore, it could be safely implemented also in agricultural applications. Keywords Basil . Cross-connection detection . Dual distribution systems . Online monitoring . ORP . Public health . Plants; s . Soil . Sulfite . Wastewater reuse
Introduction Rapid population growth coupled with ever-increasing water demand for agricultural, municipal and industrial uses has led to depletion of natural water sources and water scarcity in many regions (Henderson et al. 2009; Hambly et al. 2010; Adewumi et al. 2010). Reuse of municipal wastewater effluent can alleviate this stress on natural water sources. Recycled wastewater is being used worldwide for many purposes, including agricultural irrigation, municipal non-potable uses (such as garden and park irrigation and toilet flushing), various industrial uses and even augmentation of potable water supplies (Hambly et al. 2012). As expected, waterscarce countries and regions are leading the reuse of recycled water; for example, over 80 % of the municipal wastewater effluent in Israel is reused predominantly for agricultural irrigation; in Australia, 19 % is being reused mostly for agricultural irrigation but also for urban nonpotable uses; in Spain, 11 % is being reused mostly for agricultural irrigation and for recreational and urban uses; California (southern) reuses 13 % of its treated wastewater, 37 % of which is reused for agricultural irrigation, 17 % for landscape irrigation and 12 % for
131
Page 2 of 18
aquifer recharge; in Singapore, highly treated effluent (termed NEWater) is reused for potable supply consisting 30 % of the national water supply (Newton et al. 2010; Vanham 2011; Cohen et al. 2012; Sato et al. 2013). With the increasing reuse of treated wastewater effluent, growing emphasis is being placed on minimising public health risks, part of which is related to the distribution network of the treated effluent. Dual distribution (or separate dual-pipe) system (one for potable water and one for treated wastewater effluent) is a key in the redistribution of recycled water for reuse. Dual distribution systems must be carefully managed to protect consumers’ safety from incorrect/incidental crossconnections between potable and recycled water lines, an event which has potential detrimental public health implications. Indeed, countries derived regulations tackling this issue. The major part of these regulations relate to infrastructure (e.g. purple colour of pipes conveying reclaimed effluent, clear signs marking reclaimed water distribution systems, one-way valves, and air gaps) or restricting the use of reclaimed water in certain areas (e.g. Israel Ministry of Health (2002) requires minimum distance between agricultural plots irrigated with treated effluent and residential/public buildings and roads). Notwithstanding, in large dual distribution systems, it is impossible to ascertain that cross-connections will never occur. Indeed, hundreds of incidents of cross-connections have been reported worldwide, mainly from the USA, Europe and Australia (Oesterholt et al. 2007; Mena et al. 2008; Henderson et al. 2009; Hambly et al. 2012; Pitkanen 2013). It is estimated that many more occurred but not reported in the scientific literature. Considering the fact that cross-connections are de facto inevitable to a certain (low) extent and the consequential negative health effects, there is a need to reliably detect cross-connections online, using an indicator that can consistently distinguish between potable water and treated effluent. Despite this need, only few studies were published on the subject. These tried to differentiate between potable and recycled wastewater by measuring parameters such as electrical conductivity (EC), total organic carbon (TOC), turbidity and UV-Vis absorption (Hambly et al. 2012). Hall et al. (2007) investigated how changes in water quality parameters, which potentially indicate contamination resulting from a cross-connection event, can be detected by real- or near-real-time sensors. They revealed that sensors that responded to most contaminants were those that monitored free chlorine, TOC, oxidation reduction potential (ORP), EC and chlorides. They stated that these
Environ Monit Assess (2015) 187:131
results may be used in a warning system for contamination within a water distribution system. Henderson et al. (2009) suggested using fluorescence spectroscopy as a potential monitoring tool, basing their suggestion on publications that reported that sewage intrusion to rivers and estuaries can be identified by fluorescence spectroscopy. However, they stated that in recycled wastewater systems where the effluent is highly treated, the impact of advanced treatment processes on this monitoring tool is largely unknown and requires investigation, and therefore, advanced data analysis techniques are required to improve the ability to detect contamination events. To improve the ability to monitor and rapidly identify (real-time) cross-connections between treated effluent and potable water distribution systems, we developed a continuous active monitoring approach. The approach is based on injection of a tracer into the effluent line and identification of its effects (not the tracer itself) in the potable water line as a signal of a cross-connection event. The chosen tracer is sodium bisulfite (SBS): when injected into the effluent line reduces its ORP that is usually lower than the ORP of potable water. Monitoring is performed by continuous measurement of the ORP in the potable water pipe where a significant decrease of its value would indicate a cross-connection event. Sodium bisulfite The term Bsulfiting agents^ is used to describe sulfur dioxide (SO2) and several forms of inorganic sulfite, which liberate sulfur dioxide under certain conditions (Nair et al. 2003). The sulfur system exists in equilibrium water solutions where the dominant species are pH dependent, for example, at pH 7.4 (37 °C), a mixture of sulfite ions (SO3−2) and bisulfite anions (HSO3−) predominates. More on sulfite reactions and kinetics can be found elsewhere (Andersen 2003). Sulfiting agents are used in the food industry, primarily to reduce or prevent spoilage and discoloration, to bleach food starches, to condition dough in baked foods, to control fermentation of wine and to soften corn kernels during wet milling (Nair et al. 2003). Concentrations above 100 ppm are found in dried fruits, while lower concentrations (10–100 ppm) are found at many other food products such as frozen and dried potatoes, pickles, gravies and jams. The World Health Organization (WHO) recommended acceptable daily intake level of 0.7 mg/kg body weight. The US Food and Drug Administration (USFDA) classified sulfiting agents as Bgenerally recognised as safe^
Environ Monit Assess (2015) 187:131
and as sources of vitamin B1 (thiamine). Their concentrations in wines and raw shrimp were respectively limited to 350 ppm (equivalent to 100 ppm SO2). There are some reports of adverse reactions by asthma patients following ingestion of sulfiting agents. Therefore, the USFDA requires packaged foods containing more than 10 ppm sulfite to list it on the ingredient label. Sulfiting agents are also used in the cosmetics industry as antioxidants. The USFDA does not require reporting their concentrations in cosmetics (Nair et al. 2003). Sodium bisulfite (SBS; NaHSO3), one of the sulfiting agents, is used by the cosmetics industry (Nair et al. 2003). In agriculture, it serves as a growth regulator used, at low concentrations, for enhancing photosynthesis to promote field crop yield (Wang et al. 2003; Guo et al. 2006; Yang et al. 2008; Bian et al. 2009). Guo et al. (2006) applied SBS to strawberry plants and reported a rise in net photosynthetic rate. Yang et al. (2008) reported that spraying low concentration of SBS on tea trees significantly enhanced the net photosynthetic rate and carboxylation efficiency. Bian et al. (2009) reported that SBS can accelerate transformation of chloroacetanilide herbicides to less toxic transformation products by dechlorination in aquatic environments. To summarise, there are reports in the literature on using SBS in agriculture to increase photosynthesis leading to increased yields, and there are some indications that it does not have a negative impact on crops. Nevertheless, only a scant number of reports were found and this issue deserves further investigation. This paper describes the development of the new monitoring method. It starts with the rationale of the method. Then, results of batch experiments where various effluent sources were mixed with potable water at different dilution ratios are presented. This is followed by results of a pilotscale continuous flow system where a cross-connection was purposely made at varying dilution ratios. Since the implementation of the approach includes addition of SBS to effluent used in many cases for agricultural irrigation, the last part of the paper experimentally examines its possible effects on soil and plants.
Materials and methods The proposed approach Following the literature review, we stated three requirements from a continuous real-time monitoring approach for identification of cross-connections: (1) The
Page 3 of 18 131
monitored parameter should be present in the recycled wastewater stream and should be easy to quantify (preferably online); (2) it would be beneficial if the presence of this parameter could be increased by a controlled manner, to produce a significant difference against its possible presence in potable water; and (3) as treated effluent is often used for irrigation, the chosen parameter should not have detrimental effects on the irrigation system, soil, crops or public health. In view of this, the chosen parameter should be soluble in water; be easily identified, low cost and available allowing continuous monitoring; should not damage the distribution system (e.g. clogging, sedimentation, fouling, corrosion); should not be toxic to humans, flora and fauna; and be approved for use in food industry. As aforementioned, the parameter chosen is oxidation reduction potential (ORP). ORP describes the ratio between oxidised and reduced chemical species in a solution, being high when a solution contains more oxidised species than reduced ones, and low vice versa. Potable water generally contains low concentrations of reducing compounds and usually some level of residual chlorine (for disinfection; oxidising species); therefore, its ORP values usually should be relatively high (from ~300 to ~900 mV depending on the source of water; Suslow 2004; Edzwald and Tobiason 2011). In contrast, the ORP of treated effluent that contains reduced species (e.g. organic substances) is usually lower. Even when the effluent is chlorinated, its ORP is expected to remain low due to the chlorine demand of the reducing substances present. During a cross-connection event, the two streams (treated effluent and potable water) mix and the ORP, measured at the potable water system, decreases. Yet, dilution of the two streams is expected. Hence, especially when the proportion of effluent in the cross-connected stream is lower than the proportion of potable water and its degree of treatment is high, the ORP decrease may be difficult to identify. To overcome this obstacle, SBS (a reducing substance) is added to the effluent in order to decrease its ORP and thus increase the difference between the ORP of the potable water and the effluent making the identification of a crossconnection easier. From an operational point of view, it should be sufficient to inject SBS to the effluent irrigation system immediately when irrigation starts for a certain period of time (e.g. 15–30 min). This should give a high enough signal that could be identified in the potable water distribution network should a crossconnection exist.
131
Page 4 of 18
Experimental methods The approach was developed and tested first by batch experiments in the laboratory, then by continuous pilotscale experiments where cross-connections were intentionally created. Both stages were performed with various dilution ratios between the effluent and the potable water streams. Possible impacts (positive or negative) of SBS on soil and plants were studied in soil columns and in planted-pots experiments. Batch experiments Batch experiments were performed with effluent from seven sources varying in the degree of treatment (Table 1), comprising of wastewater treatment plants (WWTPs) effluent (EF), effluent from effluent storage reservoirs (SRs) and reclaimed water (RW, i.e. Shafdan WWTP effluent after soil aquifer treatment (SAT) and some dilution with groundwater). To simulate a crossconnection event at a particular place, samples of potable water were taken from the vicinity of each effluent source. The potable water was chlorinated to meet one of the following concentrations: 0.1, 0.5 and 1.0 mg/L residual chlorine (after 30-min contact time). SBS was added to the effluent to achieve concentrations of 10, 20 and 30 mg SO−3/L. Nine combinations from each source were examined (three chlorine levels × three SBS levels), each at six dilution ratios: 1:1, 1:2, 1:10, 1:20, 1:50 and 1:100 (1:100 being one effluent volume in 100 potable water volumes). The ORP of the potable water (unchlorinated and chlorinated), effluent (with and without SBS) and effluent-water mixtures was measured with an ORP electrode (Sensorex S200C/BNC Comb pH Electrode on Eutech CyberScan 110 pH/ORP Portable Meter). All chemical analyses were performed according to the Standard Methods (Apha et al 2012). Continuous flow pilot-scale experiments A pilot plant was constructed at Shomrat SR, which stores the secondary effluent of Acre WWTP (Table 1). All pipes used were made of PVC; thus, no reaction between chlorine added and the pipe material was expected. The pilot system consisted of two separate streams, namely, potable water and treated effluent (Fig. 1); each was fed from a separate tank. Hypochlorite could be injected into the potable water line to obtain residual chlorine concentration of 0.1 up to
Environ Monit Assess (2015) 187:131
1 mg/L (after 30-min contact time). SBS could be dosed into the effluent line to reach its desired concentrations. The ORP of each stream was continuously measured by ORP electrodes (SRH-1, Seko Ltd). The two streams were intentionally cross-connected in a designated point where a third ORP electrode was placed. The pilot plant was operated through a computerised control system which controlled all valves, pumps, chlorine and SBS dosing, and dilution ratios between the two streams. All data (including flows, dilution ratios and readings of the three ORP electrodes) was stored in a data logger every 10 s. The mixing ratio of the two streams was controlled by proportional valves and bypass streams. Prior to each experiment run, the ORP electrode was checked and calibrated with reference solution (Redox Standard, 250 mV at 25 °C Ag/AgCl, Reagecon, Ireland). Precautions were taken to avoid health risk, including installation of non-return valve on the potable water stream and an air plug on the pipe feeding the potable water tank, and discharging the mixture (after the crossconnection point) to the SR. The system was operated in two modes: 1. Short experiments that tested the system’s ability to identify a cross-connection were examined at six dilution ratios (effluent/potable water): 1:2, 1:5, 1:10, 1:20, 1:50 and 1:100. The system was operated 6 h a day at three different dilution ratios (2 h each). During the first 30 min, SBS and chlorine were not added in order to monitor background ORPs of the three streams (effluent, potable water, mixture effluent-potable). During the next 60 min, SBS was dosed to the effluent stream and chlorine to the potable water one. Then, SBS and chlorine addition was halted and the system was let to stabilise in a different dilution ratio. During the whole period, ORP values were continuously monitored in all three streams. 2. Long experiments where the system was operated with the same dilution ratio for a long period (up to 3–4 weeks) and injection of SBS and chlorine were performed intermittently in order to simulate repeated irrigation with effluent.
Impact on soil and plants Possible effects of SBS addition on soil were studied in soil columns, and on basil (Ocimum basilicum) plants grown in pots. Basil was selected as a model plant due to
Environ Monit Assess (2015) 187:131
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Table 1 Characteristics of the effluent sources used in the batch experiments and in the pilot-scale system Experiment type
Batch
Pilot-scale (continuous)
Source
Ein Hashofet Nir Ezion Ra'anana Haifa
M. Kishon Shafdan
Type
SRa
SRb
–
EF
EF
EF
2.0°105
4.3°106
3.5°107 –
Shomrat
RW before SR RW after SR SRc –
Q
m3/y
V
3
m
2.3°10
pH
–
8.1
7.5
7.1
7.8
7.2
7.3
7.3
EC
mS/cm 3.9
0.89
1.2
1.7
0.97
0.83
1.4
TSS
mg/L
5.6
7.4
14
0.40
2.2
7.8
CODd
mg/L
15
80
12
~0
78
CODt
mg/L
69
