Chemistry of wastewater

Chemistry of Wastewater- My Knowledge Gap Water and Wastewater Water is a very innocent chemical substance, containing hydrogen and oxygen. In chemistry pure water does mean 2 molecules of hydrogen are covalent with 1 molecule of oxygen. But this water never considered as drinkable water as we have more expectation from water. We not only drink water to get rid from thirstiness but also to get some very important minerals in a balance quantity and that is why the term” mineral water” is very appropriate to drinkable water. However, drinkable water should be safe enough- it should be free of unwanted ions, suspended particles or microbial contamination without which it is called wastewater. So, wastewater is nothing but the impure water containing suspended and dissolves materials in unexpected quantities.

Wastewater Chemistry When the term wastewater comes, it becomes necessary to quantify the impurities present there and for this, we used several direct and indirect methods. The chemical and biological (here I mean microbial cells) substances which makes wastewater impure are known as pollutants. As my concern is here only about the chemicals, I am not interested to discuss more about microbial pollution. So here, I categorized the pollutants present in wastewater into 2 categories:  

Macro Pollutants: present in large quantities and comparatively less harmful for human and environment but have long term environmental impact.

Micro Pollutants: present in very small quantities, has greater health and environmental impact which are either persistent or bio-accumulative or both.

I do not want to go more details about the classification of pollutants, lots of information about that is available in classical wastewater chemistry books.

But one thing, I just wanted to mention here that all pollutants in wastewater remains in two state, either Suspended or Dissolve which I am going to discuss further in another section of this paper.

Measurement of Pollutants in Wastewater  

Direct measurement: measurement of individual pollutants by modern instruments such as Chromatography and spectrophotometry (e.g. GC-Mass) Indirect Measurement: This is very popular in wastewater engineering. In this method, quantification of impurities are done in a gross scale such as Oxygen demand (BOD and COD) to quantify the organic pollutants and Salinity (TDS or EC) for inorganic pollutants (dissolve substances)

Wastewater Discharge Standards Most of the wastewater standards in the developing countries are only concern about macro pollutants and only in a few cases concern about the micro pollutants. So, the typical wastewater parameters which they set to be maintained to discharge into natural water bodies contains the following very favorite term:

a. b. c. d. e. f.

Biological Oxygen Demand (BOD) Chemical Oxygen Demand (COD) pH Total Suspended Solid (TSS) Total Dissolve Solid (TDS) And in some cases some heavy metal content and persistent organic matter content (e.g. organochlorine substances, etc.)

As I mentioned earlier, to me pollutants in wastewater are two categories, suspended and dissolve, I am in the side of measuring the macro pollutants in only by two parameters 

Total Suspended Solid Total Dissolve Solid

Yes, the other parameters like pH, individual metal contents and persistent organic matter content contents can be measures which depends on how stricter the standards is?

Chemistry of Suspended Solid Suspended solid in wastewater can be classified into several categories   

Very large floating substances Small suspended particles Fine particles Very fine particles (e.g. emulsion)

On the other hand, based on biodegradability those can be classified as  

Easily biodegradable Moderately biodegradable Persistent (degrade in geological time scale)

The above mention classification is only from my personal view, may be does not seems so important to discuss more here. Rather I am going to discuss more about suspended solid in a board.

To measures the suspended hydrocarbon in wastewater, 2 indirect measurements methods are widely used1. Biological Oxygen Demand (BOD) 2. Chemical Oxygen Demand (COD) Both of these two measurement techniques are co-related. Biochemical (in some cases it is also named biological) Oxygen Demand is the amount of oxygen taken up by the microbial cells/ colony to oxidize (metabolized) the hydrocarbon present in the wastewater. Usually it is calculated over a fixed time limit such as in 3 days period (BOD3), in 5 days period (BOD5) and in some cases the ultimate BOD value to see exactly the amount of Oxygen required to oxidize the hydrocarbon present in wastewater.

On the other hand, the term COD is the amount of oxygen required to oxidize the total carbon present in the wastewater. To me a very good oxidizer like Potassium Dichromate can oxidize the 100% of the organic matter present in the textile/ footwear wastewater. So, the following equation be writtenCOD= bCOD+ nb COD Where, b COD= the amount of oxygen required by microbial community to metabolize biodegradable organic matter, which termed as BOD nbCOD= the portion of organic matter which is not possible to metabolize by the microbial community but readily oxidizable by chemical reaction.

So, COD= BOD+nbCOD-----------------------------------------------------------------(1) And nbCOD= COD-BOD This nbCOD mainly refers the quantity of organic carbon which is not easily biodegradable by microorganism or it requires very long time to degrade. In case of effluent treatment plant, a certain portion of this organic matter can be biologically oxidized by extended aeration system (24-48 hours).

My Knowledge Gap In the beginning of this section, I just wanted to mention some parameters of Pakistan National Water Quality Standards, just as an example.    

BOD5------------------------------------ ≤ 80 PPM COD---------------------------------------≤ 150 PPM TSS----------------------------------------≤ 150 PPM TDS-----------------------------------------≤ 3500 PPM

Just for logical aspect, I would like to do some calculation hereTotal suspended solid mentioned in the above standards is 150 mg/l I hope, most of you know the bellow very simple reaction-

C+O2------------------------------- CO2

The atomic weight of Carbon is 12 where as that of Oxygen is 16. So, I can calculate like thisTo oxidize 12 gm of Carbon, it requires 32 gm of Oxygen, i.e. to Oxidize 1 gm Carbon it requires 2.66 gm Oxygen.

So, to oxidize 150 gm suspended solid, it requires- 150X2.66= 399 gm Oxygen, let’s say 400 gm.

And if TSS remains 150 mg/l, the total oxygen demand would be approximately 400 mg/l where in the standard, the total oxygen demand is only 150 mg/l (ref. equation-1; COD value itself includes the BOD value).

Now I would like to mentioned the same parameters as per Business Social Responsibilities (BSR Standards)  

BOD5------------------------------------ ≤ 30 PPM COD---------------------------------------≤ 200 PPM TSS----------------------------------------≤ 30 PPM

As per this BSR standards, to oxidize 30 gm of Suspended Organic Carbon, it requires 30X2.66= 79.8 gm Oxygen. Here I am convinced, if TSS can be maintained 30 PPM and factories do not have to be worried about BOD and COD values.

But in Bangladesh and Pakistan where TSS requirement is 150 mg/l, how they will maintain BOD and COD requirements, I cannot understand. In reverse order, it can be said that if factories are able to maintain COD value ≤ 200 (for Bangladesh) and ≤ 150 (for Pakistan), their corresponding TSS will be 75 and 56.25 mg/l only, if such than what is the meaning of keeping this TSS requirements (≤ 150 PPM) in the standards.

Conclusion Finally, I would like to request my friends who have very deep understanding about wastewater chemistry, could you please help me to be clear about this issue, first of all- am I right here? If not what is in the gap (I mean is there anything suspended in the wastewater which does not contribute in COD measurement). One thing, I could understand is - there can be some inert suspended solid such as silicon particles in wastewater which can one of the things contributing in TSS but not in COD. In that case, what are the sources of such silicon particles in wastewater in textile industry as they use very pure water (even that with O TSS). I am really helpless here, if any of you have any idea regarding this gap, I would be grateful if you write further on this, I have doubt whether we are discharging a lot of very complex persistent organic matter with wastewater which are not even oxidizable by chemical oxidizer.