TRIHALOMETHANE FORMATION IN PUBLIC DRINKING WATER SUPPLIES

Transcription

1 TRIHALOMETHANE FORMATION IN PUBLIC DRINKING WATER SUPPLIES THE DEVELOPMENT OF NEW TECHNICAL GUIDANCE FOR THE EPA Presentation to: EI West Region Lecture Programme 4 th October 2012 by MICHAEL JOYCE, & VICTOR van der WALT, Project Manager

2 PRESENTATION OVERVIEW MICHAEL JOYCE Outline of EPA project deliverables The reactivity of Natural Organic Matter (NOM) as THM precursors in chlorinated water and how best to monitor this reactivity Health risks associated with different trihalomethanes and their volatilisation to air Factors affecting THM formation in chlorinated supplies The efficacy of pre-treatment processes for THM precursor removal ahead of chlorination VICTOR van der WALT The efficacy of different existing treatment processes to remove THM precursors Review of site specific data on THM formation Methodologies for monitoring THM formation potential

3 EPA PROJECT DELIVERABLES A desk based assessment of investigation files relating to 25 public drinking water supplies where THM exceedances have occurred and preparation of individual site reports Comment on the solutions put forward by the WSAs for resolving THM issues on the 25 sample supplies Attend the site audits of 10 public drinking water supplies where frequent THM exceedances have been recorded Produce a technical guidance document on the overall findings of the assessment of the files and detail how these findings could be transferred to a policy or programme nationally to tackle THM exceedances at other supplies EPA Advice Note 4 Version - published by EPA in 2012

4 ORIGIN AND COMPOSITION OF SOURCE WATER CONTAMINANTS Organic Particulates or Natural Organic Matter (NOM) Particulate or dissolved matter resulting from degradation of plant and animal materials Inorganic Particulates Mineral particulate matter resulting from weathering, erosion or run off in the catchment e.g. silts, clays, metal oxides Pathogenic microorganisms Bacteria Viruses Protozoaon cysts such as Cryptosporidium

5 PARTICLE SIZES OF SOURCE WATER CONTAMINANTS

6 THE NEED FOR PRE-TREATMENT AHEAD OF DISINFECTION Surface waters (& surface contaminated groundwaters) (>90% of sources) require multi-barrier approach to treatment and disinfection Effective treatment upstream of chlorination can: Remove pathogens, reducing the challenge to final disinfection (particularly for pathogens such as Cryptosporidium, which is highly resistant to chemical disinfectants) Make the water more amenable to final disinfection, by reducing chlorine demand and the constituent elements of total organic carbon (TOC) turbidity, colour and dissolved organic carbon (DOC) Reduce the potential for THM by-product formation by reducing natural organic matter i.e. reduction of TOC which is the precursor to chlorinated by-product formation

7 NATURAL ORGANIC MATERIAL (NOM) NOM is a complex mixture of organic compounds resulting from the degradation of vegetative matter in the catchment Compounds resulting from the growth and decomposition of algae and weeds with the water source itself NOM is classified into two components Non-humic substances namely proteins, polysaccharides and carboxylic acids Humic substances namely humic acids and fulvic acids with molecular weights between hundreds and tens of thousands Typical Humic NOM molecule

8 THE NEED TO REMOVE NOM The removal of NOM in drinking water is required because It causes colour, taste and odour in water unacceptable to consumers; It reacts with most disinfectants used in water treatment, thus reducing their disinfection efficacy in the inactivation of pathogens; It influences disinfectant demand & disinfection process design and operation; It can prevent the verification of UV disinfection systems It affects stability and removal of inorganic particles in water; It has a large effect on coagulant demand and coagulation performance; It can cause irreversible fouling in membrane filtration systems; It competes with taste and odour for adsorption onto GAC filters thereby reducing their adsorption capacity It can affects biological re-growth in distribution systems; Chlorination of NOM produces disinfection by-products including THMs;

9 Due to the predominance of surface water sources or surface water r affected groundwater sources in Ireland, almost all water treatment facilities ities in Ireland use chlorination as part of the treatment process. CHLORINATION FOR PRIMARY AND SECONDARY DISINFECTION PRIMARY DISINFECTION The main disinfection method employed where a chemical or non-chemical disinfectant is used and has to be verified as per SI 278 of 2007as achieving the necessary microbial inactivation of pathogenic microorganisms in water Chlorination others namely UV disinfection, Ozone, Chlorine dioxide SECONDARY DISINFECTION The application of a chemical disinfectant at the end of a treatment system or at some appropriate point along the distribution network to maintain n the disinfection residual and quality assure drinking water throughout the system to the point of compliance i.e. minimum of 0.1mg/l at the consumer tap Chlorination others namely Monochloramine (Chloramination)

10 TOTAL TRIHALOMETHANES Chlorination of organic matter in drinking water Cumulative total of four constituent THMs with parametric limit of 100µg/l Chloroform (CHCl 3 ) - highest concentration Bromoform (CHBr 3 ) Dibromochloromethane (CHClBr 2 ) [DBCM] Bromodichloromethane (CHCl 2 Br) [BDCM] Chloroform and BDCM classified by the IARC as Group 2B compounds (i.e. possible carcinogen to humans) with chloroform linked to possible liver damage and BDCM linked to a possible increase in reproductive effects Bromoform and DBCM classified by the IARC as Group 3 compounds (i.e. not classifiable as carcinogen to humans)

11 CHEMICAL REACTIONS ASSOCIATED WITH CHLORINATION Free chlorine, which is formed consequent to the use of chlorination as a primary or secondary disinfectant, exists in water w as either hypochlorous acid (HOCl( HOCl) ) or hypochlorite ion ( - OCl) dependent on ph.

12 THE HOCl to OCl- EQUILIBRIUM AND ITS DEPENDENCY ON ph AND TEMP HOCl OCl - Fraction (molar) oC 15oC 35oC 5oC 15oC 35oC ph

13 CHEMICAL REACTIONS ASSOCIATED WITH CHLORINATION Free chlorine, which is formed consequent to the use of chlorination as a primary or secondary disinfectant, exists in water w as either hypochlorous acid (HOCl( HOCl) ) or hypochlorite ion ( - OCl) dependent on ph. Oxidants such as ozone or free chlorine oxidize the bromide ion to hypobromate ion/hypobromous acid In the presence of bromides in the water, brominated THMs are formed preferentially and chloroform concentrations decrease proportionally

14 VOLATILISATION OF THMs Chloroforms and BDCM are extremely volatile relative to the other two constituents of TTHMs and are ultimately transferred to air as a result of their volatility WHO research suggests equal contributions to total chloroforms and BDCM exposure coming from 4 areas: ingestion of drinking water inhalation of indoor air largely due to volatilization from drinking-water inhalation & dermal exposure during showering or bathing ingestion of food Indoor air exposure is particularly related to rates of ventilation ion in houses and frequency of showering and bathing

15 REACTIVITY OF NOM AS THM PRECURSORS IN WATER Hydrophobic fraction - primarily humic acids Humic acids and lignins, which are produced from decaying vegetative matter (tend to form higher THMs levels) Lignin is resistant to biodegradation yet reactive with oxidants owing to a high density of activated aromatic chemical rings in decaying vegetative matter Hydrophilic fraction - primarily fulvic material Carbohydrates and sugars (relatively poor THM precursors) Compounds resulting from algal activity Waters abstracted from different locations in a catchment and at different times of the year will have distinctly different mixtures of these THM precursors.

16 FACTORS AFFECTING THM FORMATION Concentration of NOM in water (i.e. TOC) and the efficacy of TOC removal by treatment Higher chlorine dose results in higher THM potential. THM formation increase with increase in ph Rate of THM formation increase as temperature increases Extended contact time. Sediments in distribution networks or reservoirs can react with residual chlorine. Ingress of surface water into reservoirs or distribution network Bromide concentration in water can result in brominated THMs

17 Seasonality in THM peaks occurs due to coincidental peaks in water temperature variation and available humic matter in early autumn In Ireland water temperature ranges from 3-18 C with the highest temperature typically recorded in late Sept/early Oct THM formation in water increases with rising temperature In autumn, the proportion of lignins and hydrophobic matter rises again due to the increased load of dying vegetation High water temperature in the distribution system promotes the accelerated depletion of free Cl 2 residual, requiring higher doses Water demands on certain schemes are often higher in summer months, resulting in lower water age helping to control THM formation during the peak summer months During summer a large portion of NOM in water is hydrophilic in nature due to algal activity and less reactive with chlorine THE SEASONALITY OF PEAK THM FORMATION IN DRINKING WATER

18 IMPORTANCE OF CONTACT TIME AND CHLORINE DOSE IN THM FORMATION The vast majority of TTHMs are formed following primary disinfection in the first 8 hours of contact with chlorine, usually in the main storage reservoir Chlorine doses and consequently THM formation are typically more significant in primary disinfection due to the higher chlorine doses required to verify disinfection efficacy where chlorine demand remains in incompletely treated water due to excessive remaining TOC in the water Following secondary chlorine disinfection, THM formation reactions become disinfectant-limited when the free chlorine residual typically drops below a free chlorine residual of 0.3mg/L Where organic matter has not been adequately removed or organic sediments exist in reservoirs and pipelines, booster chlorination facilities can result in increased THM formation

19 RELATIONSHIP BETWEEN TOC, CHLORINE DOSE AND CONTACT TIME Extract from Casey TJ and Chua HK (1995) Trihalomethanes in Drinking Water - IEI Dublin Time t from chlorine addition in hours

20 ASSESSMENT OF NOM IN WATER Surrogate parameters which facilitate the assessment of NOM & the monitoring of water treatment plant operation and performance are e : Total and dissolved organic carbon (TOC and DOC); UV absorbance at 254 nm wavelength (UVA 254 ) UV Transmittance (UVT) which is deducible from UVA 254 in accordance with UVT = UVA or UVA = 2 - Log 10 UVT UV 254 is related to the aromatic chain components of NOM and is considered a good predictor of TOC levels in water and the tendency of NOM in the water following treatment to form THMs after chlorination DOC usually constitutes > 90% of remaining TOC levels in filtered waters

21 SUVA is defined as the ratio between ultra violet absorbance (UVA) at 254nm (UV 254 ) & the DOC in the water SUVA = UVA 254 (cm - 1 ) X 100 DOC (mg/l) SUVA>4 indicates the presence of humic matter of high aromatic and hydrophobic character with high UVA (or low UVT), high chlorine demand and high THM formation potential (THMFP) 2<SUVA<4 indicates a mixture of humic and non-humic matter of both hydrophobic and hydrophilic character, medium UVA, lower chlorine demand and less THMFP SUVA< 2 indicates a high fraction of non-humic matter of hydrophilic character with low UVA, a low chlorine demand and low THMFP Guidance WHERE SUVA > 2.5, PLANT COAGULATION DOSING IS BEST CONTROLLED BY UVA 254 or TOC UVA 254 SUVA AS AN INDICATOR OF NOM TYPE AND THM FORMATION POTENTIAL (L/mg.min)

22 Because of the relatively large size of dissolved humic substance molecules, their removal from water using a sufficiently tight membrane (nano( nano-filtration) Because the humic substances are highly negatively charged at drinking water ph, they can be charge neutralised, coagulated, adsorbed to or enmeshed in metal hydroxide floc and removed by sedimentation/flotation followed by filtration Because of this negative charge, humic matter may be removed by chemical (ion exchange) or physical adsorption (activated carbon) Since colour is associated with aromatic content (C=C bonds) of larger humic substances, the color can be removed by breaking these bonds using a strong oxidant such as ozone Humic matter is generally non-biodegradable. The use of a strong oxidants like ozone can break humics into smaller, biodegradable components, removable by activated carbon or biofiltration. NOM REMOVAL PROCESSES

23 REMOVAL CAPABILITY OF WATER TREATMENT PROCESSES

24 VICTOR van der WALT

25 TOC mg/l RANGE OF TOC CONCENTRATIONS IN IRISH WATERS S u r f a c e G r o u n d Rivers and streams Upland lakes Lowland lakes Springs Shallow wells Deep wells THM Formation Potential High Medium Low Victor van der Walt Project Manager for the EPA THM project

26 SUMMARY OF TREATMENT PROCESSES IN EPA PROJECT ON THM FORMATION IN PWSS No Pre- Treatment Direct Filtration Slow Sand Filtration Ozone + GAC 9 Victor van der Walt Conventional Treatment Project Manager for the EPA THM project

27 SUMMARY OF TTHM (max) TTHM max TTHM (ug/l) CT 1 CT 2 CT 3 CT 4 CT 5 CT 6 CT 7 DF 1 NPT 1 NPT 2 NPT 3 NPT 4 NPT 5 NPT 6 NPT 7 OZ + GAC Oz + SSF SSF 1 SSF 2 SSF 3 SSF 4 SSF 5 SSF 6 SSF 7 SSF 8 No Pre-Treatment Direct Filtration Slow Sand Filtration Ozone + Slow Sand Filtration Ozone + GAC Conventional Treatment Victor van der Walt Project Manager for the EPA THM project

28 SUMMARY OF TTHM vs TW TOC TTHM (ug/l) RW TOC (mg/l) 0 0 CT 1 CT 2 CT 3 CT 4 CT 5 CT 6 CT 7 DF 1 NPT 1 NPT 2 NPT 3 NPT 4 NPT 5 NPT 6 NPT 7 OZ + GAC Oz + SSF SSF 1 SSF 2 SSF 3 SSF 4 SSF 5 SSF 6 SSF 7 SSF 8 No Pre-Treatment Direct Filtration Slow Sand Filtration TTHM max RW TOC max Ozone + Slow Sand Filtration Ozone + GAC Conventional Treatment Victor van der Walt Project Manager for the EPA THM project

29 TTHM (ug/l) THM FORMATION INCREASE AFTER BOOSTER CHLORINATION Case Study SFF1 Increase in THM formation due to secondary booster chlorination Without effective treatment for NOM removal, THM formation in excess of parametric limits will continue to ensue following primary, secondary and booster chlorination. Primary chlorine dose A1 Secondary chlorine dose 20 0 Jan-11 Feb-11 Apr-11 May-11 Jun-11 Jul-11 Aug-11 Sep-11 Oct-11 Nov-11 Dec-11 Victor van der Walt A1 A2 B1 B2 A2 Project Manager for the EPA THM project B2 B1

30 TTHM (ug/l) THM FORMATION INCREASES WITH RISE IN ph Case Study SFF Jan-10 Feb-10 Mar-10 Apr-10 May-10 Jun-10 Jul-10 Aug-10 Oct-10 Dec-10 Feb-11 9 ph TTHM ph ph adjustment should be provided to optimise chlorine disinfection at a ph 7.5. Victor van der Walt Project Manager for the EPA THM project

31 SLOW SAND GAC SANDWICH ADSORPTION vs BIOLOGICAL Case Study SFF1 New slow sand filter with GAC sandwich commissioned on 17 Aug 2011 TTHM (ug/l) GAC becomes exhausted and TOC passing through slow sand filter increases 50 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Victor van der Walt Project Manager for the EPA THM project

32 Source Protection Case Study NPT 5 (No pre-treatment) 2004 to 2007 Spring source under the direct influence of surface water WSA completed source protection works to exclude all surface water ingress to to 2010 TTHM <5-109 <6 TOC <1 Victor van der Walt Project Manager for the EPA THM project

33 OZONE + GAC FILTRATION vs CONVENTIONAL TREATMENT Treatment TOC (mg/l) DOC (mg/l) UVA SUVA (L/mg.min) TOC removal efficiency TTHM (mg/l) (reservoir outlet) Ozone and GAC (GAC operating as a biological filter) RW TW % 86 Coagulation, sedimentation and filtration RW TW % 69 While final water colour less than 20 Hazen may be indicative of the visual acceptability of drinking water to consumers, it is not an a indication of the DOC fraction of TOC containing most of the THM precursors. (NB. Oxidation does not remove organic contaminants) Victor van der Walt Project Manager for the EPA THM project

34 TOC REMOVAL EFFICIENCY ULTRAFILTRATION MEMBRANES RW Alkalinity < 60 mg/l as CaCO 3 RW Alkalinity > 120 mg/l as CaCO 3 TOC (ug/l) UR 1 UR 2 UR 3 UL 1 UL 2 LL 1 LL 2 LL 3 LL 4 UR upland river UL upland lake RW TOC TW TOC LL lowland river Victor van der Walt Project Manager for the EPA THM project

35 7 GWS + 1 PWSS - Reported THM Exceedances Quality standard in accordance with Part 1, Table B of S.I. No.278 of Apr-11 May-11 Jul-11 Sep-11 Oct-11 Nov-11 Dec-11 Feb-12 Apr-12 Jun-12 Jul-12 Aug-12 Membrane 1 Membrane 2 Membrane 3 Membrane 4 Ozone 1 Ozone 2 Ozone 3 Large PWSS Victor van der Walt Project Manager for the EPA THM project

36 Victor van der Walt Project Manager for the EPA THM project

37 Victor van der Walt Project Manager for the EPA THM project

38 Victor van der Walt Project Manager for the EPA THM project

39 Victor van der Walt Project Manager for the EPA THM project

40 TOC REMOVAL EFFICIENCY CONVENTIONAL TREATMENT Case Study 1 Coagulation, sedimentation and filtration Treatment Nov Aug RW TW RW TW TOC (mg/l) DOC (mg/l) ph UVA SUVA (L/mg.min) TOC removal target (Alk < 60) 50% 50% TOC removal efficiency 60% 67% TOC monthly performance ratio TTHM (mg/l) (reservoir outlet) Victor van der Walt Project Manager for the EPA THM project

41 TOC REMOVAL EFFICIENCY CONVENTIONAL TREATMENT Case Study 2 Coagulation, sedimentation and filtration Treatment Jun Jul RW TW RW TW TOC (mg/l) DOC (mg/l) Alkalinity (mg/l) UVA SUVA (L/mg.min) TOC removal target 35% 25% TOC removal efficiency TOC monthly performance ratio TTHM (mg/l) (reservoir outlet) Victor van der Walt Project Manager for the EPA THM project

42 Objective : reduce THMs Options : remove DOC or change disinfectant POC WTP Titanic DOC