Chlorine dioxide

Biodegradation in water and sediment: simulation tests

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

biodegradation in water: sewage treatment simulation testing

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Principles of method if other than guideline:

Chlorine dioxide was produced on site with a Prominent generator by mixing sodium chlorite (6-8 %) and HCl (10 %) in a chamber containing Raschig rings. The produced chlorine dioxide was subsequently diluted with distilled water in a second mixing chamber and added to the sewage flow before premixing tank. The actual concentration of chlorine dioxide produced by the generator and the presence of other chemicals (chlorite, chlorate and chlorine) were measured before and after every ClO2 disinfection test.
The testing was carried out in a pilot plant installed in a municipal wastewater-treatment plant located in Genoa (Italy) and administered by AMGA S.p.A. The municipal plant (about 220,000 equivalent inhabitants corresponding to a maximum load of 26,400 Kg COD/day) uses a conventional sewage-treatment system based on screening, aeration in grit chamber and biological oxidation through activated sludge, secondary clarification, and chlorination.
The pilot plant was fed by the effluent from the secondary settler placed after the activated-sludge biological tank of the municipal plant. Residence times (tR) in the pilot plant were determined at chosen values (1.0 - 6.0 m3/h) of sewage flow rate Q by injecting rapidly a tracer (about 5 L of a saturated solution of sodium chloride) into the sewage stream at the inlet of the premixing tank and by measuring the electrical conductivity at the exit of the contact basin.

GLP compliance:

not specified

Radiolabelling:

no

Oxygen conditions:

aerobic/anaerobic

Inoculum or test system:

other: Municipal waste water

Initial conc.:

0.5 mg/L

Based on:

act. ingr.

Initial conc.:

0.9 mg/L

Based on:

act. ingr.

Initial conc.:

1.7 mg/L

Based on:

act. ingr.

Initial conc.:

3.6 mg/L

Based on:

act. ingr.

Parameter followed for biodegradation estimation:

test mat. analysis

Details on study design:

The testing, organized by AMGA S.p.A., Caffaro S.p.A and Istituto Superiore di Sanità, was carried out in a pilot plant installed in a municipal wastewater-treatment plant located in Genoa (Italy) and administered by AMGA S.p.A.
The municipal plant (about 220,000 equivalent inhabitants corresponding to a maximum load of 26,400 Kg COD/day) uses a conventional sewage-treatment system based on screening, aeration in grit chamber and biological oxidation through activated sludge, secondary clarification, and chlorination.
The pilot plant (Figure 1) consisted of a stainless steel structure made up of a premixing chamber (35 cm long, 25 cm wide and 58 cm high), a contact basin (200 cm long, 70 cm wide and 89 cm high) and a final weir, which set liquid level (about 1.0 m3) in the contact basin.
It was fed by the effluent from the secondary settler placed after the activated-sludge biological tank of the municipal plant. Table A7_1_2_2_1-1 lists physico-chemical, chemical and microbiological composition of the sewage employed during the experimentation. The effluent sucked by a centrifugal pump was introduced in the premixing tank containing a stirrer (350 rpm) and directly connected to the contact basin subdivided crosswise by seven septa (70 cm x 70 cm) with staggered openings (10 cm x 59 cm). The flow rate (1.0 - 10.0 m3/h) was regulated by a throttle valve and checked with a volume meter both installed immediately after the feeding pump.
Residence times (tR) in the pilot plant were determined at chosen values (1.0 - 6.0 m3/h) of sewage flow rate Q by injecting rapidly a tracer (about 5 L of a saturated solution of sodium chloride) into the sewage stream at the inlet of the premixing tank and by measuring the electrical conductivity at the exit of the contact basin.

Duration of the test: Average reaction times of 14, 23 or 37 minutes

Reference substance:

not required

% Degr.:

100

Parameter:

test mat. analysis

Sampling time:

37 min

Remarks on result:

other: No chlorine dioxide residue was found in the disinfected sewage after the contact times investigated. This finding is consistent with the sewage ClO2 demand (6.9±0.7 mg/L) which was always greater than the initial concentration of the biocide introduced.

Transformation products:

yes

No.:

#1

No.:

#2

No.:

#3

Details on transformation products:

Figures 2-4 show the residual concentrations of chlorite, chlorate and total chlorine in sewage treated with chlorine dioxide and the corresponding contribution from the ClO2 generator as disinfectant dosages increased in the sewage.
The analysis of Figure 2 suggests that about 74 % of the initial ClO2 concentration in the sewage was reduced to chlorite (line slope: 0.74; standard error: 0.05) while the fraction introduced by the generator was negligible (1.2 %; standard error: 0.1 %).
On the other hand, the presence of ClO3- in the disinfected effluent (Figure 3) can be attributed exclusively to the contribution made by the generator (line slopes: 0.10 and 0.09 for total ClO3- and generator fraction, respectively; standard error: 0.01 for both regression lines).
Total residual chlorine detected in wastewater treated with chlorine dioxide (Figure 4) was low. The fraction introduced by the generator was about 23 % (standard error: 3 %) of the residual concentration in the sewage disinfected with ClO2.

Details on results:

No chlorine dioxide residue was found in the disinfected sewage after the contact times investigated. This finding is consistent with the sewage ClO2 demand (6.9±0.7 mg/L) which was always greater than the initial concentration of the biocide introduced.

Results with reference substance:

Not applicable

Validity criteria fulfilled:

not applicable

Conclusions:

This study shows that chlorine dioxide is rapidly degraded in sewage treatment wastewater, when the biological demand is greater than the applied dose of the biocide.

Reference 2

Endpoint:

biodegradation in water: simulation testing on ultimate degradation in surface water

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Principles of method if other than guideline:

An investigation into the degradation of chlorine dioxide in aqueous systems was performed using water from three sources – industrial wastewater, surface (river) water and tap (drinking) water. Chlorine dioxide (0, 1, or 3 mg/L) was added to 500 mL of water in a flask which was then Stoppered and stirred. Samples of the solution were removed at 0, 5, 10, 30 and 60 minutes and 20 hours and the level of chlorine dioxide residual measured using a spectrophotometric method at 340 nm. The pH of the test waters was between 7.0 – 7.9 and the temperature was 22 °C.

GLP compliance:

not specified

Radiolabelling:

no

Oxygen conditions:

aerobic/anaerobic

Inoculum or test system:

other: Industrial effluent, surface water and tap water

Details on source and properties of surface water:

Industrial effluent: Waste water from a tissue mill. Sample was taken just before being dispatched in the recipient (river Göta Älv).
Surface water: The River Göta Älv which supplies the Gothenburg Drinking water works with water. Sample was taken in the river outside EKA Chemicals
Tap water: Drinking water from the city of Gothenburg, disinfected with monochloramine (< 0.3 mg/L as Cl2)

Initial conc.:

0 mg/L

Based on:

test mat.

Details on study design:

Industrial effluent: Waste water from a tissue mill. Sample was taken just before being dispatched in the recipient (river Göta Älv).
Surface water: The River Göta Älv which supplies the Gothenburg Drinking water works with water. Sample was taken in the river outside EKA Chemicals
Tap water: Drinking water from the city of Gothenburg, disinfected with monochloramine (< 0.3 mg/L as Cl2)

Duration of the test: 0, 5, 10, 30, 60 minutes or 20 hours

Reference substance:

not required

% Degr.:

ca. 100

Parameter:

test mat. analysis

Sampling time:

5 min

Remarks on result:

other: In wastewater effluent, an initial dose of 3 mg/L chlorine dioxide was completely reacted after 5 minutes contact time.

Compartment:

other: Tap water

DT50:

27.7 h

Type:

not specified

Remarks on result:

other: 1 mg/L initial ClO2 concentration

Compartment:

other: surface water

DT50:

16 min

Type:

not specified

Remarks on result:

other: 1 mg/L initial ClO2 concentration

Compartment:

other: surface water

DT50:

22 min

Type:

not specified

Remarks on result:

other: 3 mg/L initial ClO2 concentration

Transformation products:

not specified

Details on results:

In wastewater effluent, an initial dose of 3 mg/L chlorine dioxide was completely reacted after 5 minutes contact time.
In surface (river) water, residual chlorine dioxide was detected in the test solution 60 minutes after addition of either 1 or 3 mg/L. No residual chlorine dioxide was detected in the 3 mg/L test solution after 20 hours contact time.
In tap water (drinking water) the initial dose of 1 mg/L was reduced to 0.6 mg/L after 20 hours contact time.

No standard deviations available

Validity criteria fulfilled:

not applicable

Conclusions:

Although selective, chlorine dioxide reacts with a number of inorganic and organic substances like iron, sulphur compounds (organic as well as inorganic), phenolic compounds and humus acids.
Every surface water, ground water, waste water etc. is unique regarding composition of substances that can react with chlorine dioxide. The lab study has consequently to be seen as an example of how chlorine dioxide may decay in the aqueous environment.
No decay of ClO2 could be detected using tap water during the evaluated time frame. The reason for the slow decay in tap water is the low amount of substances that can be oxidized. Still a low amount of ClO2 in the water leaving the water plant is desired in order to prevent recontamination of the water and to avoid bio fouling of the water pipes.

Reference 3

Endpoint:

biodegradation in water: sewage treatment simulation testing

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Principles of method if other than guideline:

Testing was performed in the disinfection basin of a conventional activated sludge plant. The chlorine dioxide demand of wastewater entering the basin was measured daily during the study. Chlorine dioxide was produced on site starting from sodium chlorite 25 % w/w solution and hydrochloric acid 33 % with a new concept generator at a production capacity in the range 2000-5000 g/h: the chlorine dioxide production was linked to the water to be treated flow rate to maintain the desired dosage (0.9-1.1 mg/L). Disinfected wastewater was collected at the end of the basin and analysed for residual chlorine dioxide and disinfection by-products.

GLP compliance:

not specified

Radiolabelling:

no

Oxygen conditions:

aerobic/anaerobic

Inoculum or test system:

other: Municipal waste water

Duration of test (contact time):

ca. 18 min

Initial conc.:

0.9 - 1.1 mg/L

Based on:

test mat.

Parameter followed for biodegradation estimation:

test mat. analysis

Details on study design:

Chlorine dioxide was dosed through a diffuser into the wastewater from a conventional activated sludge plant at the entrance to the disinfection basin.
The disinfected water was sampled at the exit of the disinfection basin in clean and sterilized glass bottles: for microbiological analyses sodium thiosulphate was added to destroy any possible chlorine dioxide residue. Many analyses were carried out directly in the plant laboratory. The disinfectant residue was determined immediately after the sampling. All the analyses were, at any rate performed within 24 hours, adopting, for the different parameters, all the procedures required by the analytical methods in terms of sample stabilization.
Each day during the trial water was sampled at the entrance of the disinfection basin and chlorine dioxide demands were performed during the period 12 April – 4 May. The residual chlorine dioxide was determined according to the CPR method (Chlorophenol red method – UNICHIM method.77; I.J.Fletcher, P.Hemmings, Determination of chlorine dioxide in potable water using Chlorophenol red”, Analyst, 1985) dosing 8 mg/L of chlorine dioxide from a stock pure chlorine dioxide 1025 mg/L (prepared in lab according to Standard Methods for the examination of water and wastewater,19th ED, 1995, met. 4500-ClO2 B- Iodometric Method). The residual chlorine dioxide was checked at 15, 30, 45 and 60 minutes and the chlorine dioxide demand calculated.

Reference substance:

not required

% Degr.:

100

Parameter:

test mat. analysis

Sampling time:

18 min

Remarks on result:

other: No residual chlorine dioxide was detected at the exit of the disinfection basin on any of the test days.

Transformation products:

no

Details on results:

No residual chlorine dioxide was detected at the exit of the disinfection basin on any of the test days.
The chlorine dioxide demand of water entering the disinfection basin varied between 4.4 and 7.1 mg/L during the study.

Table A7_1_2_2_1-2:        Chlorine dioxide demand vs. Time, pH : 6,5-7, temperature 18-21 °C

Time	CHLORINE DIOXIDE DEMAND
	measure unit	12-April	13-April	19-April	28-April	4 May
15 min	mg/L	4,4	4,7	4,6	4,85	6,18
30 min	mg/L	5,4	5,15	5,2	5,68	6,8
45 min	mg/L	5,9	5,6	5,5	5,79	7
60 min	mg/L	5,95	6	5,75	5,98	7,1

Table A7_1_2_2_1-3:        Chemical and microbiological data of the water after the chlorine dioxide disinfection

Date		12-apr	13-apr	19-apr	19-apr	28-apr	04-may
Sampling time		16,20	10,30	11,30	14,30	11,30	10,30
Chlorine dioxide dosage		0,9	1,0	1,0	1,1	1,0	1,1
pH		6,5	6,5	6,75	6,45	6,5	6,93
Redox potential	mV	298	320	195	200	339	260
conductivity	mS/cm	1275	1200	1026	1056	987	1283
oxygen	mg/L	4	4,3	3,7	3,5	4,6	3
colour	Pt/Co	53	45	47	49	27	83
turbidity	NTU	2	1,9	2	2,1	2,3	3,5
Chlorine dioxide residue	mg/l	nr	nr	nr	nr	nr	nr
Active chlorine	mg/L	0,04	0,05	0,03	0,02	0,02	0,03
COD	mg/L	18	20	25	29	8	25
TOC	mg/L	13	11,5			11	12,5
DOC	mg/L	11,5	11			10,6	12
UV 254 nm	Abs/cm	0,217	0,19	0,212	0,212	0,226	0,187
DUV 254 nm	Abs/cm	0,193	0,178	0,204	0,196	0,215	0,168
fluoride	mg/L	0,15	0,14	0,13	0,16	0,12	0,15
chlorite	mg/L	0,52	0,59	0,56	0,59	0,53	0,54
chloride	mg/L	121	110	103	102	95,2	112,5
nitrite	mg/L	0,13	0,14	0,19	0,12	0,34	0,59
bromide	mg/L	0,17	0,13	0,1	0,11	0,12	0,11
chlorate	mg/L	0,07	0,03	0,07	0,07	0,03	0,05
nitrate	mg/L	18,3	12,8	18,5	22,9	19,7	3,75
sulphate	mg/L	107	110	96	96	90,1	109,8
sodium	mg/L	108	109	91	93,6		101,5
ammonia	mg/L	9,9	14,4	6	6		17,2
potassium	mg/L	16,6	16,7	12,9	12,9		15,7
magnesium	mg/L	18,2	18,4	15,6	15,7		16,6
calcium	mg/L	101	99,4	84	85,5		91
TTHMs	mg/L	1,2	1,2	0,7	1	1,1	1,4
CHCl3	mg/L	1,2	1,2	0,7	1	1,1	1,4
CHCl2Br	mg/L	n.r.	n.r.	n.r.	n.r.	n.r.	n.r.
CHBr2Cl	mg/L	n.r.	n.r.	n.r.	n.r.	n.r.	n.r.
CHbr3	mg/L	n.r.	n.r.	n.r.	n.r.	n.r.	n.r.
Esch. coli	cfu/100 mL	120	400	20	23	50	150
Coli. tot	cfu/100 mL	1,6*10^3	1,8*10^4	4*10^3	4*10^3	7,6*10^2	3,5*10^3
Salmonella spp		nd	nd	Nd	nd	nd	Nd
Acute toxicity Bioluminescent  bacteria		No tox	No tox	No tox	No tox	No tox	No tox

nr = not detectable

Validity criteria fulfilled:

not applicable

Additional information

Studies carried out on secondary effluents demonstrate that chlorine dioxide has a half-life from seconds to minutes under laboratory conditions. As degradation is abiotic (in contact with organic matter and oxidisable material), a guideline simulation test is inappropriate.