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Reverse osmosis concentrate disposal in the UK
DESALINATION Desalination 132 (2000) 47-54
ELSEVIER
i
www.elsevier.com/locate/desal
Reverse osmosis concentrate disposal in the UK Deborah Squire Anglian Water Innovation, Thorpe Wood House, Thorpe Wood, Peterborough, PE3 6WZ, United Kingdom Tel. +44 (1733) 414361; Fax +44 (1733) 414350; e-mail: [email protected]
Received 22 June 2000; accepted 14 July 2000
Abstract
Over the past 10-15 years there has been a world-wide increase in the number and size of water treatment plants utilising reverse osmosis (RO) membrane technology for the production of potable water [1]. As membrane plants continue to increase both in size and quantity, so will the potential impact of RO concentratedisposal on the environment. The issue of concenWatedisposal must be addressed as an integral part of the design and evaluation of any membrane process and the method of disposal will ultimately be site specific. The costs of disposal and any treatment can have a considerable impact on the overall economic viability of a membrane plant. This paper will discuss the issues pertinent to RO concenlrate disposal in the UK along with an emphasis on factors conlributingto the chemical characteristicsof RO concentrate. Keywords: RO concentrate disposal; Concentrate characteristics
1. Introduction
Over the past 10-15 years there has been a world-wide increase in the number and size of water treatment plants utilising reverse osmosis (RO) membrane technology for the production of potable water [1]. As membrane plants continue to increase both in size and quantity, so will the potential impact of RO concentrate disposal on the environment. In a typical potable water application treating groundwater, RO filtration is used to treat a proportion of the total volume while the remainder is treated using conventional methods
(Fig. 1). RO membranes typically retain >95% of dissolved ions so the permeate produced is not used directly as potable water. In order to achieve the desired dilution of the target determinands the permeate is blended with the conventionally-treated stream. The selection of blend ratio and conventional treatment method will depend on the raw water characteristics and the final quality requirements. The symbiotic relationship between the new technology, RO filtration, and conventional potable water production methods is illustrated by this type of process integration. Where product quality requirements become more
Presented at the Conferenceon Membranesin Drinkingand Industrial Water Production,Pads, France, 3-6 October 2000 InternationalWaterAssociation,EuropeanDesalinationSociety,AmericanWaterWorksAssociation,JapanWaterWorksAssociation 0011-9164/00/$-- See front matter© 2000 ElsevierScienceB.V. All fights reserved PII: S00I 1 - 9 1 6 4 ( 0 0 ) 0 0 1 3 4 - X
D. Squire/Desalination 132 (2000) 47-54
48
Oxidation/ Aeration/ Chlorination Contact tank
Sand f i l t r a t i o n
Cartridge filtration
Anti-scalant
RO filtration
RO concentrate
Fig. 1. Schematic of an example of potable water productionusing split-stream RO filtration.
stringent and source water quality deteriorates, RO filtration can play an ever growing part in enhancing and prolonging the working life of many conventional potable water producing treatment works. The issue of concentrate disposal must be addressed as an integral part of the design and evaluation of any membrane process and the method o f disposal will ultimately be site specific. The costs of disposal and any treatment can have a considerable impact on the overall economic viability of a membrane plant. In the UK, the disposal of RO concentrate from Water Treatment Works can be achieved by a variety of routes such as the following: • • • •
direct disposal to receiving water indirect disposal to receiving water disposal to wastewater (foul sewerage) system reuse as irrigation water
The Environment Agency is the regulatory body in the UK with respect to surface water
discharge; and discharge to wastewater system is subject to trade effluent regulations. 2. RO concentrate characteristics
The chemical characteristics of RO concentrate have a profound influence on the viability of the available modes of disposal. Unlike many conventional water treatment processes, for example lime softening, RO membrane filtration does not produce more pollutant material or mass in the waste stream but redistributes what is present in the feed water. RO concentrate also differs from most process wastewaters as the dissolved species present are not ostensibly characterised by process added chemicals but by the species present in the raw water [2]. The chemical characteristics of RO concentrate reflect the RO feed water source. However, within the confines of the nature of the RO feed water there is a certain amount of control which can be exercised regarding the specific charac-
D. Squire/Desalination 132 (2000) 47-54
teristics of the concentrate which will represent the final effluent requiring disposal. The influencing factors include: • •
RO system pre-treatment selection RO system recovery
Throughout the RO filtration process, the permeate product water is separated continuously and thus the final concentrate is a product of the cumulative rate of concentration of dissolved ions. Any sparingly soluble salts in the feed water are prevented from precipitating during the filtration process by RO pre-treatment. An example of pre-treatment may be pH correction or the addition of a scale inhibitor chemical. The pre-treatment methodology is crucial to the performance of the RO system and along with the mode of operation will influence the characteristics of the concentrate. 2.1. RO pre-treatment influences on concentrate characteristics'
RO pre-treatment can substantially influence the concentrate characteristics so the selection will have ramifications on the feasibility of means of concentrate disposal. RO filtration is a process which requires a minimum quality of feed water in order for filtration to take place effectively and for the system as a whole to perform efficiently. The feed water must be: •
•
particulate-free able to be modified to prevent precipitation during the concentration process within the RO unit
Thus, there are minimum requirements for pre-treatment. Beyond these limitations, there is scope for additional pre-treatment to influence the concentrate characteristics. The use of additional pre-treatment will represent an extra operating cost, but will be cost-effective if a means of concentrate disposal is facilitated.
49
Particulate removal can be as simple as cartridge filtration but more extensive treatment such as sand filtration or ultraflltration may be required. Pre-treatment to suppress precipitation of inorganic salts can be the addition of acid and/or anti-scalant. Acid addition will lower the feed water pH and influence the carbonate equilibria of the concentrate. Anti-scalant addition does not affect the chemical balance of the feed water in this way but maintains the concentrate as a stable supersaturated solution. Typically, anti-scalant pre-treatment involves the addition of relatively low concentrations of chemicals (usually less than 10 mg/l) so the concentrate will substantially reflect the constituents of the raw water [3].
2.2. RO system recovery influence on concentrate characteristics
In order for RO treatment to be economically viable for groundwater sources in the type of split-stream potable water production illustrated in Fig. 1, a high system volume recovery is necessary. The system volume recovery is the volume of permeate produced from the feed water volume expressed as a percentage. High recovery leads to a concentrating effect of dissolved species in the feed water, the extent of which can be estimated from the following mass balance derived equation. CF -
1
1-R
(1)
where C F - concentration factor of ionic species; R - system recovery expressed as a decimal. Fig. 2 illustrates the increase in concentration factor effect at higher system recoveries (also shown in Table 1) derived using Eq. (1). It can be seen that even a slight change in system recovery, especially above 80%, can have a profound effect on the concentrate
50
D. Squire/Desalination 132 (2000) 47-54 i00
i
8O 60-
r..)
20
0
20
I
I
40
60
80
i00
Systlml volrne% mszx)vezy
Fig. 2. RO systemrecoveryeffecton concentrationfactor. Table 1 Concentration factor as a function of system recovery Percentage recovery
Concentration factor
33 50 67 75 80 85 90 95 97.5 98 99
1.5 2 3 4 5 6.7 10 20 40 50 100
Table 2 Effect of system recovery on concentratecharacteristics
A good approximation of concentrate quality with respect to concentration factor can be predicted when designing the RO system using software based on fundamental theory and data values for the known membrane material and system configuration. The impact o f selection of RO system recovery is shown in the following example. Table 2 shows typical data used to design a membrane system where RO splitstream treatment has been selected as the appropriate treatment for a high-nitrate groundwater source. The projected feed source nitrate levels over the 10 year expected life of the RO plant were estimated at between 55 and 60 mg/l nitrate as NO3. Data in Table 2 shows that the concentrate quality with respect to nitrate level varies considerably for an RO plant operated at 80% recovery at the current feed source nitrate level of 50 mg/l nitrate as NO3 to that at 85% recovery at the projected feed source nitrate level of 60 mg/l nitrate as NO3. The concentrate nitrate levels for these 2 operating scenarios are 245 and 390 mg/l nitrate as NO3 respectively. Evidently, the 2 system recoveries give rise to concentrate volumes of, respectively, 20% and 15% o f the RO feed volume. This example illustrates the need to consider concentrate volume and quality ramifications on potential disposal methods which, once in place, will need to be sustainable for the lifetime of the RO plant.
Concentrate nitrate, mg/l N-NO3 Feed water nitrate, mg/I as NO3 50 55 60
80% system recovery
85% system recovery
3. RO concentrate disposal
245 269 294
325 357 390
The most viable options for concentrate disposal, which may or may not involve further treatment, are as follows:
characteristics. Therefore, it is imperative to consider concentrate characteristics with respect to potential disposal options at the initial RO system design stage.
• • •
direct discharge to receiving water indirect discharge to receiving water disposal to wastewater (foul sewerage) system • reuse as irrigation water
D. Squire/Desalination 132 (2000) 47-54 4. Direct discharge to receiving water Discharge to a receiving surface water, directly or indirectly, is one of the most common method of disposal owing to the availability, cost-effectiveness and mixing volume associated with a receiving water body. The main concerns with disposal of RO concentrate are to prevent detrimental effects on the aquatic environment of the receiving water and contamination by undesirable ionic species of not only current potable water supplies, but also to future reclaimable water supplies. In some cases, the concentrate can be of an equal or higher quality than the water with which it will eventually be mixed [4].
4.1. UK regulatory requirements for surface water discharge The regulatory body involved in discharge consent consideration in the UK is the Environment Agency although other organisations such as the Royal Society for the Protection of Birds (RSPB) or English Nature may have interests in specific sites. Direct discharge of an RO concentrate to a surface water in England and Wales requires compliance with a legal consent set by the Environment Agency [5]. A consent to discharge is a legal document and applies to a point source discharge, that is, specific, identifiable discharges of effluent from a known location. As such, a discharge consent must be issued for concentrate disposal to a surface water, for which an annual fee is payable to the EA. The role of the EA is to protect and improve the water environment in England and Wales [6]. The EA aims to achieve a continuing overall improvement in the quality of rivers, estuaries, coastal waters and groundwaters, through the control of pollution, as well as ensuring that the dischargers pay the costs of the consequences of their discharges.
51
Within its regulatory framework, the EA must ensure that the correct environmental and discharge standards are set and enforced. The discharge of effluent to a receiving water is acceptable providing the quality of that receiving water consistently meets or exceeds the minimum requirements set by the EA. A useful guide to the initial scoping of any scheme which may affect the water environment is the EA's Environmental Assessment: Scoping Handbook for Projects [7] which aims to encourage early consultation in proposals. Where split-stream RO treatment is used to produce potable water, there may be other site effluents which are disposed of via a discharge consent to a surface water. This effluent is potentially available to be blended with the RO concentrate to form a combined discharge compliant with an EA discharge consent which the concentrate alone would fail to meet. Effluents of this type resulting from conventional treatment processes include settled filter backwash water and GAC adsorber backwash water. An important consideration is the nature of the flows of the two process streams to be blended. RO filtration produces a constant stream of concentrate whereas backwash effluents are produced on a batch basis. To ensure compliance with a combined discharge consent, RO concentrate should not be allowed to discharge without prior blending so additional storage capacity on-site may be required. This in turn may lead to additional pumping costs depending on the method of conveyance to the point of discharge. The potential for subsequent scaling of any transfer pumps would depend on the characteristics of the blend, the retention time and the storage environment.
5. Indirect discharge to surface water In England and Wales, RO concentrate is classified in legislation as a trade effluent. The
52
D. Squire/Desalination 132 (2000) 47-54
factors influencing the feasibility of discharge to a wastewater system, as with direct discharge to surface water, are site specific. Technical and economic considerations include: • the volume of concentrate relative to the total treatment capacity of the wastewater plant (dilution factor) • the composition of the concentrate •
any charges imposed by the wastewater treatment facility
•
any effect of the concentrate on wastewater treatment equipment including the sewage treatment processes and piping to the works
•
potential effects on the wastewater facility final effluent
Examples of the potential adverse effects on sewage treatment processes include precipitation of calcium carbonate on trickling filters from a concentrate containing high levels of total hardness; and concentrate with high concentrations of nitrate may cause rising in primary sewage settlement tanks through denitrification. These effects will be dependant on the concentrate characteristics and dilution factor at the wastewater treatment facility. For membrane plants located near to wastewater treatment facilities, concentrate disposal to a wastewater system can be a very viable option. As concentrate is classified as a trade effluent when disposed of via a wastewater system, discharge requires the permission of the local water company (Sewage Undertaker). The Sewage Undertaker evaluates trade effluent and issues a discharge consent specifying volume and quality limits. Anglian Water operates a Trade Effluent Code of Practice [8] which strives to ensure consistency of trade effluent evaluation. In some circumstances where, particular organic species or heavy metals are present in the trade effluent, the Environment Agency become involved in the discharge consent application assessment.
6. Spray irrigation Concentrate can be disposed of by land application via spray irrigation. This involves transporting the concentrate to a vegetative area and utilising it as irrigation water. In a system such as this, the main mechanism for disposal is vegetation uptake with the quantities of uptake varying widely according to the crop planted. Using the concentrate in lieu of fresh irrigation water helps conserve natural resources and is especially attractive in areas where water conservation is of great importance. Environmental regulations must be closely examined to determine whether spray irrigation might contaminate groundwater or receiving surface waters. An important factor for disposal as irrigation water is that there must be a need for irrigation water in the vicinity of the membrane plant [9]. There must also be a backup disposal method or storage facility available during periods of heavy rainfall.
7. Selection of disposal option The environmental implications of concentrate disposal must be carefully considered. The determination of the method of concentrate disposal is site specific and several factors influence the choice, including: • volume of concentrate • quality or constituents of concentrate • availability of receiving site • physical or geographical location of site in relation to concentrate discharge point • dilution factors from other on-site discharges and/or the receiving water • public acceptance • capital and operating costs • ability for the facility to be expanded • site specific discharge consent conditions • regulatory requirements
D. Squire/Desalination 132 (2000) 47-54
In the UK, the application of RO technology to potable water production is still in its infancy. There have not been any surveys carried out of trends of means of concentrate disposal in the UK but in 1993, a survey was conducted at membrane drinking water plants in the continental United States of size greater than 95 m3/day [1]. 73% of the plants were brackish water RO, 11% were nanofiltration (NF), 11% electrodialysis (ED) and the remaining 5%, seawater RO plants. The survey showed that the distribution of the disposal options was as follows: • 48% Surface water • 23% Headworks of a water treatment plant • 13% Land application (including percolation) • 10% Deep well injection • 6% Evaporation ponds All of the land application sites and all but one of the deep well disposal sites were in Florida, reflecting the climate and geological conditions of that area. For the plants surveyed, disposal to surface water was the preferred option with regard to relatively larger plant size. Disposal to the headworks of a water treatment plant was associated more with smaller sized plants, reflecting the limited capacity of many treatment plants to take large volume discharges. The disposal options of land application and evaporation ponds required sufficient acreage and in some cases, appropriate groundwater and soil characteristics. This option was therefore limited to smaller plants and by the availability of land. Deep well injection was, where geologically possible, used primarily with larger sized plants. A follow up survey to this work was carried out in 1999 [10]. The percentage distribution of disposal options was similar to the previous survey as many of the same plants were resurveyed although an increase in surface water disposal to 60% of the plants responding was observed.
53
8. Conclusions In order for RO filtration to be integrated successfully into existing and future water treatment works, a thorough consideration of issues surrounding the concentrate disposal options should be carried out at the earliest possible time. The chemical characteristics of RO concentrate will be reflective of the raw water source. RO pre-treatment and the mode of RO system operation will influence the concentrate characteristics. Education is a prime objective. The perception of RO concentrate as a 'toxic waste' should be pre-empted by discussion with the pertinent bodies and individuals involved in RO projects, such as, designers, operators and Regulatory bodies. RO concentrate disposal will always be site specific but some water sources will give rise to characteristics which will invariably be problematic with respect to disposal. If no disposal method is immediately available, treatment will be required to facilitate disposal otherwise the feasibility of the scheme as a whole will be compromised. This area should be investigated. As with disposal itself, the cost implications of concentrate treatment will be site specific.
References [1] M. Mickley, R. Hamilton, L. Gallegos, and J. Truesdall, Membrane Concentrate Disposal. AWWA, ISBN 0-89867-710-6, 1993. [2] M. Mickley, Regulation and disposal of membrane concentrate in the United States, International Workshop: Membranes in Drinking Water Production, Paris, 1995. [3] M. Mickley, Desalination and Water Reuse, 5 (4) (1995). [4] P.J. Malaxos and O.J. Morin, Desalination, 78 (1990) 27.
54
D. Squire/Desalination 132 (2000) 47-54
[5] Discharge consent and compliance policy: A
[6]
blueprint for the future, National Rivers Authority (Environment Agency), Water Quality Series No. l, July 1990. Discharge Consents and Compliance: The NRA's Approach to Control of Discharges to Water, Report of the National Rivers Authority, Water Quality Series No. 17, March 1994.
[7]
Environmental Assessment: Scoping Handbook for projects, Report of the Environment Agency, April 1996. [8] Anglian Water Trade Effluent Code of Practice, Internal document. [9] W.J. Conlon, Waterworld News, 5 (1) (1989) 18. [10] E. Kenna and A.K. Zander, Survey of membrane concentrate reuse and disposal, AWWA Research Foundation Project No. 498-97, 1999.
ELSEVIER
i
www.elsevier.com/locate/desal
Reverse osmosis concentrate disposal in the UK Deborah Squire Anglian Water Innovation, Thorpe Wood House, Thorpe Wood, Peterborough, PE3 6WZ, United Kingdom Tel. +44 (1733) 414361; Fax +44 (1733) 414350; e-mail: [email protected]
Received 22 June 2000; accepted 14 July 2000
Abstract
Over the past 10-15 years there has been a world-wide increase in the number and size of water treatment plants utilising reverse osmosis (RO) membrane technology for the production of potable water [1]. As membrane plants continue to increase both in size and quantity, so will the potential impact of RO concentratedisposal on the environment. The issue of concenWatedisposal must be addressed as an integral part of the design and evaluation of any membrane process and the method of disposal will ultimately be site specific. The costs of disposal and any treatment can have a considerable impact on the overall economic viability of a membrane plant. This paper will discuss the issues pertinent to RO concenlrate disposal in the UK along with an emphasis on factors conlributingto the chemical characteristicsof RO concentrate. Keywords: RO concentrate disposal; Concentrate characteristics
1. Introduction
Over the past 10-15 years there has been a world-wide increase in the number and size of water treatment plants utilising reverse osmosis (RO) membrane technology for the production of potable water [1]. As membrane plants continue to increase both in size and quantity, so will the potential impact of RO concentrate disposal on the environment. In a typical potable water application treating groundwater, RO filtration is used to treat a proportion of the total volume while the remainder is treated using conventional methods
(Fig. 1). RO membranes typically retain >95% of dissolved ions so the permeate produced is not used directly as potable water. In order to achieve the desired dilution of the target determinands the permeate is blended with the conventionally-treated stream. The selection of blend ratio and conventional treatment method will depend on the raw water characteristics and the final quality requirements. The symbiotic relationship between the new technology, RO filtration, and conventional potable water production methods is illustrated by this type of process integration. Where product quality requirements become more
Presented at the Conferenceon Membranesin Drinkingand Industrial Water Production,Pads, France, 3-6 October 2000 InternationalWaterAssociation,EuropeanDesalinationSociety,AmericanWaterWorksAssociation,JapanWaterWorksAssociation 0011-9164/00/$-- See front matter© 2000 ElsevierScienceB.V. All fights reserved PII: S00I 1 - 9 1 6 4 ( 0 0 ) 0 0 1 3 4 - X
D. Squire/Desalination 132 (2000) 47-54
48
Oxidation/ Aeration/ Chlorination Contact tank
Sand f i l t r a t i o n
Cartridge filtration
Anti-scalant
RO filtration
RO concentrate
Fig. 1. Schematic of an example of potable water productionusing split-stream RO filtration.
stringent and source water quality deteriorates, RO filtration can play an ever growing part in enhancing and prolonging the working life of many conventional potable water producing treatment works. The issue of concentrate disposal must be addressed as an integral part of the design and evaluation of any membrane process and the method o f disposal will ultimately be site specific. The costs of disposal and any treatment can have a considerable impact on the overall economic viability of a membrane plant. In the UK, the disposal of RO concentrate from Water Treatment Works can be achieved by a variety of routes such as the following: • • • •
direct disposal to receiving water indirect disposal to receiving water disposal to wastewater (foul sewerage) system reuse as irrigation water
The Environment Agency is the regulatory body in the UK with respect to surface water
discharge; and discharge to wastewater system is subject to trade effluent regulations. 2. RO concentrate characteristics
The chemical characteristics of RO concentrate have a profound influence on the viability of the available modes of disposal. Unlike many conventional water treatment processes, for example lime softening, RO membrane filtration does not produce more pollutant material or mass in the waste stream but redistributes what is present in the feed water. RO concentrate also differs from most process wastewaters as the dissolved species present are not ostensibly characterised by process added chemicals but by the species present in the raw water [2]. The chemical characteristics of RO concentrate reflect the RO feed water source. However, within the confines of the nature of the RO feed water there is a certain amount of control which can be exercised regarding the specific charac-
D. Squire/Desalination 132 (2000) 47-54
teristics of the concentrate which will represent the final effluent requiring disposal. The influencing factors include: • •
RO system pre-treatment selection RO system recovery
Throughout the RO filtration process, the permeate product water is separated continuously and thus the final concentrate is a product of the cumulative rate of concentration of dissolved ions. Any sparingly soluble salts in the feed water are prevented from precipitating during the filtration process by RO pre-treatment. An example of pre-treatment may be pH correction or the addition of a scale inhibitor chemical. The pre-treatment methodology is crucial to the performance of the RO system and along with the mode of operation will influence the characteristics of the concentrate. 2.1. RO pre-treatment influences on concentrate characteristics'
RO pre-treatment can substantially influence the concentrate characteristics so the selection will have ramifications on the feasibility of means of concentrate disposal. RO filtration is a process which requires a minimum quality of feed water in order for filtration to take place effectively and for the system as a whole to perform efficiently. The feed water must be: •
•
particulate-free able to be modified to prevent precipitation during the concentration process within the RO unit
Thus, there are minimum requirements for pre-treatment. Beyond these limitations, there is scope for additional pre-treatment to influence the concentrate characteristics. The use of additional pre-treatment will represent an extra operating cost, but will be cost-effective if a means of concentrate disposal is facilitated.
49
Particulate removal can be as simple as cartridge filtration but more extensive treatment such as sand filtration or ultraflltration may be required. Pre-treatment to suppress precipitation of inorganic salts can be the addition of acid and/or anti-scalant. Acid addition will lower the feed water pH and influence the carbonate equilibria of the concentrate. Anti-scalant addition does not affect the chemical balance of the feed water in this way but maintains the concentrate as a stable supersaturated solution. Typically, anti-scalant pre-treatment involves the addition of relatively low concentrations of chemicals (usually less than 10 mg/l) so the concentrate will substantially reflect the constituents of the raw water [3].
2.2. RO system recovery influence on concentrate characteristics
In order for RO treatment to be economically viable for groundwater sources in the type of split-stream potable water production illustrated in Fig. 1, a high system volume recovery is necessary. The system volume recovery is the volume of permeate produced from the feed water volume expressed as a percentage. High recovery leads to a concentrating effect of dissolved species in the feed water, the extent of which can be estimated from the following mass balance derived equation. CF -
1
1-R
(1)
where C F - concentration factor of ionic species; R - system recovery expressed as a decimal. Fig. 2 illustrates the increase in concentration factor effect at higher system recoveries (also shown in Table 1) derived using Eq. (1). It can be seen that even a slight change in system recovery, especially above 80%, can have a profound effect on the concentrate
50
D. Squire/Desalination 132 (2000) 47-54 i00
i
8O 60-
r..)
20
0
20
I
I
40
60
80
i00
Systlml volrne% mszx)vezy
Fig. 2. RO systemrecoveryeffecton concentrationfactor. Table 1 Concentration factor as a function of system recovery Percentage recovery
Concentration factor
33 50 67 75 80 85 90 95 97.5 98 99
1.5 2 3 4 5 6.7 10 20 40 50 100
Table 2 Effect of system recovery on concentratecharacteristics
A good approximation of concentrate quality with respect to concentration factor can be predicted when designing the RO system using software based on fundamental theory and data values for the known membrane material and system configuration. The impact o f selection of RO system recovery is shown in the following example. Table 2 shows typical data used to design a membrane system where RO splitstream treatment has been selected as the appropriate treatment for a high-nitrate groundwater source. The projected feed source nitrate levels over the 10 year expected life of the RO plant were estimated at between 55 and 60 mg/l nitrate as NO3. Data in Table 2 shows that the concentrate quality with respect to nitrate level varies considerably for an RO plant operated at 80% recovery at the current feed source nitrate level of 50 mg/l nitrate as NO3 to that at 85% recovery at the projected feed source nitrate level of 60 mg/l nitrate as NO3. The concentrate nitrate levels for these 2 operating scenarios are 245 and 390 mg/l nitrate as NO3 respectively. Evidently, the 2 system recoveries give rise to concentrate volumes of, respectively, 20% and 15% o f the RO feed volume. This example illustrates the need to consider concentrate volume and quality ramifications on potential disposal methods which, once in place, will need to be sustainable for the lifetime of the RO plant.
Concentrate nitrate, mg/l N-NO3 Feed water nitrate, mg/I as NO3 50 55 60
80% system recovery
85% system recovery
3. RO concentrate disposal
245 269 294
325 357 390
The most viable options for concentrate disposal, which may or may not involve further treatment, are as follows:
characteristics. Therefore, it is imperative to consider concentrate characteristics with respect to potential disposal options at the initial RO system design stage.
• • •
direct discharge to receiving water indirect discharge to receiving water disposal to wastewater (foul sewerage) system • reuse as irrigation water
D. Squire/Desalination 132 (2000) 47-54 4. Direct discharge to receiving water Discharge to a receiving surface water, directly or indirectly, is one of the most common method of disposal owing to the availability, cost-effectiveness and mixing volume associated with a receiving water body. The main concerns with disposal of RO concentrate are to prevent detrimental effects on the aquatic environment of the receiving water and contamination by undesirable ionic species of not only current potable water supplies, but also to future reclaimable water supplies. In some cases, the concentrate can be of an equal or higher quality than the water with which it will eventually be mixed [4].
4.1. UK regulatory requirements for surface water discharge The regulatory body involved in discharge consent consideration in the UK is the Environment Agency although other organisations such as the Royal Society for the Protection of Birds (RSPB) or English Nature may have interests in specific sites. Direct discharge of an RO concentrate to a surface water in England and Wales requires compliance with a legal consent set by the Environment Agency [5]. A consent to discharge is a legal document and applies to a point source discharge, that is, specific, identifiable discharges of effluent from a known location. As such, a discharge consent must be issued for concentrate disposal to a surface water, for which an annual fee is payable to the EA. The role of the EA is to protect and improve the water environment in England and Wales [6]. The EA aims to achieve a continuing overall improvement in the quality of rivers, estuaries, coastal waters and groundwaters, through the control of pollution, as well as ensuring that the dischargers pay the costs of the consequences of their discharges.
51
Within its regulatory framework, the EA must ensure that the correct environmental and discharge standards are set and enforced. The discharge of effluent to a receiving water is acceptable providing the quality of that receiving water consistently meets or exceeds the minimum requirements set by the EA. A useful guide to the initial scoping of any scheme which may affect the water environment is the EA's Environmental Assessment: Scoping Handbook for Projects [7] which aims to encourage early consultation in proposals. Where split-stream RO treatment is used to produce potable water, there may be other site effluents which are disposed of via a discharge consent to a surface water. This effluent is potentially available to be blended with the RO concentrate to form a combined discharge compliant with an EA discharge consent which the concentrate alone would fail to meet. Effluents of this type resulting from conventional treatment processes include settled filter backwash water and GAC adsorber backwash water. An important consideration is the nature of the flows of the two process streams to be blended. RO filtration produces a constant stream of concentrate whereas backwash effluents are produced on a batch basis. To ensure compliance with a combined discharge consent, RO concentrate should not be allowed to discharge without prior blending so additional storage capacity on-site may be required. This in turn may lead to additional pumping costs depending on the method of conveyance to the point of discharge. The potential for subsequent scaling of any transfer pumps would depend on the characteristics of the blend, the retention time and the storage environment.
5. Indirect discharge to surface water In England and Wales, RO concentrate is classified in legislation as a trade effluent. The
52
D. Squire/Desalination 132 (2000) 47-54
factors influencing the feasibility of discharge to a wastewater system, as with direct discharge to surface water, are site specific. Technical and economic considerations include: • the volume of concentrate relative to the total treatment capacity of the wastewater plant (dilution factor) • the composition of the concentrate •
any charges imposed by the wastewater treatment facility
•
any effect of the concentrate on wastewater treatment equipment including the sewage treatment processes and piping to the works
•
potential effects on the wastewater facility final effluent
Examples of the potential adverse effects on sewage treatment processes include precipitation of calcium carbonate on trickling filters from a concentrate containing high levels of total hardness; and concentrate with high concentrations of nitrate may cause rising in primary sewage settlement tanks through denitrification. These effects will be dependant on the concentrate characteristics and dilution factor at the wastewater treatment facility. For membrane plants located near to wastewater treatment facilities, concentrate disposal to a wastewater system can be a very viable option. As concentrate is classified as a trade effluent when disposed of via a wastewater system, discharge requires the permission of the local water company (Sewage Undertaker). The Sewage Undertaker evaluates trade effluent and issues a discharge consent specifying volume and quality limits. Anglian Water operates a Trade Effluent Code of Practice [8] which strives to ensure consistency of trade effluent evaluation. In some circumstances where, particular organic species or heavy metals are present in the trade effluent, the Environment Agency become involved in the discharge consent application assessment.
6. Spray irrigation Concentrate can be disposed of by land application via spray irrigation. This involves transporting the concentrate to a vegetative area and utilising it as irrigation water. In a system such as this, the main mechanism for disposal is vegetation uptake with the quantities of uptake varying widely according to the crop planted. Using the concentrate in lieu of fresh irrigation water helps conserve natural resources and is especially attractive in areas where water conservation is of great importance. Environmental regulations must be closely examined to determine whether spray irrigation might contaminate groundwater or receiving surface waters. An important factor for disposal as irrigation water is that there must be a need for irrigation water in the vicinity of the membrane plant [9]. There must also be a backup disposal method or storage facility available during periods of heavy rainfall.
7. Selection of disposal option The environmental implications of concentrate disposal must be carefully considered. The determination of the method of concentrate disposal is site specific and several factors influence the choice, including: • volume of concentrate • quality or constituents of concentrate • availability of receiving site • physical or geographical location of site in relation to concentrate discharge point • dilution factors from other on-site discharges and/or the receiving water • public acceptance • capital and operating costs • ability for the facility to be expanded • site specific discharge consent conditions • regulatory requirements
D. Squire/Desalination 132 (2000) 47-54
In the UK, the application of RO technology to potable water production is still in its infancy. There have not been any surveys carried out of trends of means of concentrate disposal in the UK but in 1993, a survey was conducted at membrane drinking water plants in the continental United States of size greater than 95 m3/day [1]. 73% of the plants were brackish water RO, 11% were nanofiltration (NF), 11% electrodialysis (ED) and the remaining 5%, seawater RO plants. The survey showed that the distribution of the disposal options was as follows: • 48% Surface water • 23% Headworks of a water treatment plant • 13% Land application (including percolation) • 10% Deep well injection • 6% Evaporation ponds All of the land application sites and all but one of the deep well disposal sites were in Florida, reflecting the climate and geological conditions of that area. For the plants surveyed, disposal to surface water was the preferred option with regard to relatively larger plant size. Disposal to the headworks of a water treatment plant was associated more with smaller sized plants, reflecting the limited capacity of many treatment plants to take large volume discharges. The disposal options of land application and evaporation ponds required sufficient acreage and in some cases, appropriate groundwater and soil characteristics. This option was therefore limited to smaller plants and by the availability of land. Deep well injection was, where geologically possible, used primarily with larger sized plants. A follow up survey to this work was carried out in 1999 [10]. The percentage distribution of disposal options was similar to the previous survey as many of the same plants were resurveyed although an increase in surface water disposal to 60% of the plants responding was observed.
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8. Conclusions In order for RO filtration to be integrated successfully into existing and future water treatment works, a thorough consideration of issues surrounding the concentrate disposal options should be carried out at the earliest possible time. The chemical characteristics of RO concentrate will be reflective of the raw water source. RO pre-treatment and the mode of RO system operation will influence the concentrate characteristics. Education is a prime objective. The perception of RO concentrate as a 'toxic waste' should be pre-empted by discussion with the pertinent bodies and individuals involved in RO projects, such as, designers, operators and Regulatory bodies. RO concentrate disposal will always be site specific but some water sources will give rise to characteristics which will invariably be problematic with respect to disposal. If no disposal method is immediately available, treatment will be required to facilitate disposal otherwise the feasibility of the scheme as a whole will be compromised. This area should be investigated. As with disposal itself, the cost implications of concentrate treatment will be site specific.
References [1] M. Mickley, R. Hamilton, L. Gallegos, and J. Truesdall, Membrane Concentrate Disposal. AWWA, ISBN 0-89867-710-6, 1993. [2] M. Mickley, Regulation and disposal of membrane concentrate in the United States, International Workshop: Membranes in Drinking Water Production, Paris, 1995. [3] M. Mickley, Desalination and Water Reuse, 5 (4) (1995). [4] P.J. Malaxos and O.J. Morin, Desalination, 78 (1990) 27.
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[5] Discharge consent and compliance policy: A
[6]
blueprint for the future, National Rivers Authority (Environment Agency), Water Quality Series No. l, July 1990. Discharge Consents and Compliance: The NRA's Approach to Control of Discharges to Water, Report of the National Rivers Authority, Water Quality Series No. 17, March 1994.
[7]
Environmental Assessment: Scoping Handbook for projects, Report of the Environment Agency, April 1996. [8] Anglian Water Trade Effluent Code of Practice, Internal document. [9] W.J. Conlon, Waterworld News, 5 (1) (1989) 18. [10] E. Kenna and A.K. Zander, Survey of membrane concentrate reuse and disposal, AWWA Research Foundation Project No. 498-97, 1999.