Is free chlorine existed in system? And ClO- may interfere COD, even you add enoughscreenbig agent

Yes, there's free chlorine present in the samples, but this doesn't seem to affect the TOC. I've tested this by adding different concentrations of Cl- to a regular sample of the same wastewater plant. The results where nothing out of the ordinary.

I am aware that chlorine interferes with COD, but they check the amount of chlorine with a different method. Based on that Cl- value, they add a certain amount of HgSO4 to bind the chlorine.

Free chlorine affects COD, leading to inaccurate COD/TOC, and free chlorine will affect BOD, too.

But how does that explain the increase in TC, decrease in TIC and thus the increase of TOC (see added photo)? As far as we know TOC is not affected by chlorine so there must be a different explanation. I'm still guessing bacteria (sludge) which was present in off-spec those samples.

What about sulfides ferrous iron and nitrides?

Whey also can increase COD.

Also, do you have ORP data on these samples?

COD

chemical oxygen demand

TOC

total organic contents

I did not measure sulfides ferrous iron and nitrides. Could you explain me how these interfere?

How does whey interferes COD?

Also what ORP? Is that the Reduction Potential?

COD analyses is based on oxidation of EVERYTHING by chromate that is present in your sample. Thus any substances that can be oxidized including sulfides and forrous iron (II) thus increasing COD.

By the way I would sugests you to look at the calibration of your TOC/TC analyser.

Regards

interesting

Bromide in water samples is also known to interfere with COD tests.

TOC is total organic CARBON (the difference between TC (total carbon) and IC (inorganic carbon). There is also a distinct difference in the IC content, which makes me wonder if there is a change in pH.

According to Standard Methods for the Examination of Water and Wastewater (APHA, 2017), the interferents for COD are:

- Chloride (Cl-) is the most common interferent (from 2,000 mg/L Cl–, since it reacts with Ag+ and precipitates as AgCl, and thus inhibits its catalytic activity). Also bromide (Br-), iodide (I-), and any other reagent that inactivates the Ag+ can interfere similarly (halide interferences may be removed by precipitation with Ag+ and filtration before digestion).

- Nitrite (NO2-) exerts a COD of 1.1 mg O2/mg N-NO2-.

- Reduced inorganic species such as ferrous iron (Fe2+), sulfide (S2-), manganous manganese (Mn2+), etc., since they consume dichromate in the oxidation (leading to a false higher COD value).

COD is an indirect measure, and may be subject to inaccuracies and interferences, such as those mentioned above. Furthermore, the production of some recalcitrant organic product (a polymerization reaction, for example) must also be considered, since they raise the COD.

About the Total, Inorganic and Organic Carbons (TC, IC, OC, respectively), which are direct measures, perhaps your hypothesis that some bacteria is converting inorganic to organic is reasonable indeed. However, there was a very expressive increase in the organic, that needs more accurate verification.

In addition to Caroline comments I would suggest to measure pH and ORP for the samples before analyzing them for COD and TC/TOC.

Again, calibration of the instruments (colorimeter for COD and TC/TOC analyzer) maybe helpful. Sometimes you can expect drift when analysing samples with substantial differences in measurements.

First of all I'd like to apologize for not replying any sooner. It's been quite busy for me. I appreciate all of your help though!

As for Sándor's question, I'm using a catalytic combustion measurement for the determination of TOC. First TC is measured and after acidification the TIC is measured (TOC = TC - TIC).

I've done some more tests for free chloride (Cl-) but it doesn't seem to affect the TOC (which I expected).

I will do some more tests for ferrous iron (Fe2+), pH and I will also take a look at the amount of particles. Perhaps there's a relation between the percentage of particles and the COD/TOC.

I still have one question: how does the acidity (pH) affect the TOC?

I've noticed that TOC-values tend to deviate when pH is 8 or higher, especially at pH12.. A pH of 2 seems to have a higher TOC value because the TIC

Article Toxicological Effects of Disinfections Using Sodium Hypochlo...

Abstract

Sodium hypochlorite (NaOCl) is often used for disinfecting hospital wastewater in order to prevent the spread of pathogenicmicroorganisms, causal agents of nosocomial infectious diseases. Chlorine disinfectants in wastewater react with organic matters, giving riseto organic chlorine compounds such as AOX (halogenated organic compounds adsorbable on activated carbon), which are toxic for aquaticorganisms and are persistent environmental contaminants. The aim of this study was to evaluate the toxicity on aquatic organisms of hospitalwastewater from services using NaOCl in pre-chlorination. Wastewater samples from the infectious and tropical diseases department of ahospital of a large city in southeast of France were collected. Three samples per day were collected in the connecting well department at 9a.m., 1 p.m. and 5 p.m. during 8 days from 13 March to 22 March 2001, and a mixture was made at 6 p.m. with the three samples in order toobtain a representative sample for the day. The toxicity test comprised the 24-h EC50onDaphnia magnaand a bioluminescence assay usingVibrio fischeriphotobacteria. Fecal coliforms and physicochemical analyses such as total organic carbon (TOC), chloride, AOX, totalsuspended solids (TSS) and chemical oxygen demand (COD) were carried out. Wastewater samples highlighted considerable acute toxicityonD. magnaandV. fischeriphotobacteria. However, low most probable numbers (MPN), ranging from < 3 to 2400 for 100 ml, weredetected for fecal coliforms. Statistical analysis, with a confidence interval of 95%, gave a strong linear regression assessed withr= 0.98between AOX concentrations and EC50(TU) on daphnia. The identification of an ideal concentration of NaOCl in disinfecting hospitalwastewater, i.e. its non-observed effect concentration (NOEC) on algae andD. magna, seems to be a research issue that could facilitate thecontrol of AOX toxicity effects on aquatic organisms. Therefore, it would be necessary to moniator the biocide properties of NaOCl on fecalcoliforms at various doses and its toxicity effects on aquatic organisms. D2004 Elsevier Ltd. All rights reserved.

Keywords:Sodium hypochlorite; AOX; Hospital effluents; Toxicity; Daphni

The information reported in the literature gives a COD/TOC ratio of 3 frequently found in many wastewaters (Seiss et al., 2001). Gray and Becker (2002) reported a semi-empirical equation to determine the ratio between COD expressed in

mg O2/l and TOC in mg C/l.

COD = 2.67 TOC

Emmanuel et al. (2004) have found in hospital wastewater COD/TOC ratios ranged from 2.01 to 4.26. According to those authors The difference could be due to the effect of the presence of inorganic oxidizable substances in COD concentrations. According to Sponza (2003), the biodegradability of organic substances occurs as a function of the speed and completeness of its biodegradability by microorganisms . Therefore, BOD5/COD and COD/TOC ratios could be used to analyze the difficulty or ease of degradation of organic substances. "In the study on hospital wastewater, when comparing the results with the information reported in the literature, the data obtained from the COD/TOC ratio can be arranged in two groups: (i) COD/ TOC from 2.01 to 3.00, and (ii) COD/TOC from 3.01 to 4.26. In the first group, the degradation of organic substances by microorganisms would occur without difficulty; however, in the second group the substances would be difficult to degrade" (Emmanuel et al, 2004).

References

Emmanuel, E., Keck, G., Blanchard, J. M., Vermande, P., & Perrodin, Y. (2004). Toxicological effects of disinfections using sodium hypochlorite on aquatic organisms and its contribution to AOX formation in hospital wastewater. Environment international, 30(7), 891-900.

Gray SR, Becker NSC. Contaminant flows in urban residential water systems.

Urban Water 2002; 4:331–346.

Seiss M, Gahr A, Niessner R. Improved AOX degradation in UVoxidative

wastewater treatment by dialysis with nanofiltration membrane. Water

Res 2001; 13:3242–3248.

Sponza DT. Application of toxicity tests into discharges of the pulp-paper

industry in Turkey. Ecotoxicol Environ Saf 2003; 54:74–86.