Black water is wastewater from toilets and is contaminated with disease carrying bacteria. It requires adequate handling as it can generate environmental disturbances and serious health problems to human and aquatic life. These impact begins when blackwater contaminate raw water sources used for daily basis. So, black water must be processed before discharging it into river. Generally, it is stored into septic tank or directly channeled to the sewage system and then processed into treatment plant.

In this study, blackwater treatment is performed by using different systems. The observation is focused on decrease in Chemical Oxygen Demand (COD) during processing. COD is the amount oxygen required to decompose the organic matter. Also, sustainability of plants in black water is to be observed.


  1. To compare the three systems used for treatment –
    • Flood and drain system
    • System with bubbler
    • system without bubbler
  2. To check the effect of black water on plant growth

06/04/2022 – 07/04/2022

Selection of plant

After discussion, plants like water hyacinth, Canna Indica are selected. This plants can absorb pollutants and can provide suitable environment for aerobic bacteria to remove various impurities.

Fig. 1 Selected plants for treatment

Collection of black water

Firstly, for collecting black water a tank is placed at drainage outlet of girls hostel, which is primarily treated in K.K.Nag’s Bio-digester. On second day, water was collected in the tank.

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Fig. 2 Collected black water


Determination of COD

Chemical oxygen demand is amount of dissolved oxygen that must be present in water to oxidize chemical organic materials. The initial COD of black water is needed to determine. Sample for determining COD collected from collected black water in the tank. The procedure used for it by using COD digester is as follows:

Materials for COD test

  1. Potassium dichromate (K2Cr2O7).
  2. Mercuric sulphate (HgSO4)
  3. Sulfuric acid (H2SO4)
  4. Ferrous ammonium sulphate (FAS)
  5. Ferron indicator
  6. Potassium hydrogen phosphate (KHP)
  7. 250 ml Erlenmeyer flask
  8. Reflux condenser
  9. COD tubes
  10. Measuring cylinder
  11. Analytical balance
  12. COD tube stands
  13. Hand gloves

Preparation of chemicals

  1. Preparation of 0.25 N potassium dichromate solution: –
    12.25 gm. potassium dichromate powder dissolved in 1000 ml distilled water in 1000
    ml volumetric flask.
  2. Preparation of Ammonium Ferrous Sulphate solution: –
    24.5 gm. Ammonium Ferrous sulphate powder dissolved in 250 ml distilled water in
    250 ml distilled water in 250 ml volumetric flask. Add 5ml Sulphuric acid.
  3. Preparation of Potassium Hydrogen Phosphate solution: –
    0.425 gm. of dried KHP powder dissolved in 1000 ml distilled water in 1000 ml
    volumetric flask. (Dried this powder for 2 hours in oven)


  1. Took 0.4 gm. AgSo4 (silver sulphate) in COD tubes.
  2. Added 20 ml sample in each COD tube.
  3. Added 20 ml distilled water in each COD tube.
  4. Added stones (crush on beads) 1 or 2 pieces.
  5. Added 10 ml (0.25 N) K2Cr2O7 Potassium dichromate solution.
  6. Added 30 ml conc sulfuric acid slowly along with swirling.
  7. If the solution turns green more known quantities of K2Cr2O7 solution is needed to add.
  8. Kept COD tubes in COD apparatus to digest the sample..
  9. Attached the condenser & set 150 Degree Celsius temperature.
  10. Kept for 2 hours filled COD tubes in COD apparatus for 150 degree Celsius.
  11. After 2 hours switched it off & removed COD tubes & cooled sample for room temperature.
  12. Took the above sample in a conical flask. Added 150 ml distilled water.
  13. Titration: – Titrated above solution with 0.1 N FAS (ferrous ammonium sulphate) solution by using Ferron indicator (add 3-4 drops). The end point of this titration is blue green to reddish brown.
Fig. 3 Performing experiment for COD determination

Formula of COD

COD = {(a-b) X N X 8000}/ml of sample solution

  1. A-Reading blank solution.
  2. B-Reading with sample solutions
  3. N- Normality of FAS solution

Dilution factor = ( ml of sample + distilled water )/ ml of sample

Calculation of COD

SampleReadingsCOD (mg/lit)
Black water8.12340
Grey water8.9612
Table no. 1 Calculation of COD of day 1
SampleReadingsCOD (mg/lit)
Tap water9.561.2
Black water8.51632
Grey water9.4714
Table no. 2 Calculation of COD of day 2

The sample of grey water and black water is diluted with distilled water and dilution factor is considered in calculation. On first day, sample of grey water was collected from grey water system near Dixit Sir’s house and on second day, it was collected from grey water system near the food lab. The COD of black water is 2340 mg/lit and we have to reduce it.


Collection of material

The material required for setup of systems for treatment of black water is as follows:

  1. Buckets
  2. Bubbler
  3. Stand
  4. Lateral
  5. T-joint
  6. Elbow
  7. Grommet
  8. wires
  10. Sub-mersible pump 18 watt


Bill of material

After calculating the initial COD, the set-up of all systems i.e. bubbling and without bubbling, flood and drain is done.

  • Bubbling system: In this system, bubblers are used which will aerate the water and provide oxygen to bacteria which will further decompose organic matter. The two buckets are used in which 17 litres of black water is filled. In first bucket, water hyacinth is planted and in second indian shot is planted.
  • Without bubbling system: This system is same as bubbling system. Only difference is bubblers are not used.
Fig.4 Process flowchart for bubbling system
Fig.5 2D CAD design for bubbling system
Fig.6 Process flowchart for without bubbling system
Fig.7 2D CAD Design of without bubbling system
Fig. 8 Preparation of set-up for bubbling and without bubbling systems
  • Flood and drain system: In this system, the bacteria will get more space to grow and the media used i.e. bricks will also give some nutrients for growth of bacteria. Contact of bacteria with air will be more as water will flood and drain after fixed time of interval. The setup is using two buckets and two reservoirs.. Buckets are placed on stand and reservoirs are under the stand.The bucket in which Indian shot is planted is connected with drip pipe at bottom. Hence, the black water will drain out from bottom. And the bucket in which water hyacinth is planted is connected with drip pipe at a height of 5 cm from side. This drip pipes are connected with submersible pumps. A timer is used for this system and the interval is set for every 30 minutes.. Both pumps used are operating on single arduino timer.
Fig.9 Process flowchart for flood and drain system
Fig.10 2D CAD design of flood and drain system

Block diagram

Fig. 11 Block diagram for connection of pump
Fig.12 Connection diagram for pump
Fig no. 13 Process flowchart for Arduino connection

Arduino Program for setting timer

int relay1 = 4;
int relay2 = 5;

void setup() {
// put your setup code here, to run once:
pinMode (relay1, OUTPUT);
pinMode (relay2, OUTPUT);

void loop() {
// put your main code here, to run repeatedly:
digitalWrite(relay1,LOW);//relay on
delay(110000);//1 MIN 83SEC
digitalWrite(relay1,HIGH);//relay off
delay (1800000);//30MIN
Fig. 14 Preparation of Set-up for flood and drain system
Fig. 15 Setup of flood and drain system


Observations of COD(1st trial)

without bubblerwith bubblerflood and drain
Initial COD (day 1) (mg/lit)234023402340
Initial COD (day 1) (mg/lit)163216321632
After 17 days (mg/lit)183021421326
Table no. 3 COD Readings of 1st trial

Due to unavailability of chemicals the COD is determined after 17 days. It is needed to determine after fixed days of interval to see the effect of different systems and plants.

Fig. 16 Plants in black water system


Trial 2nd

The three systems of black water were setup on different times and the results cannot be compared effectively. So, the systems are set again at same time. For this, the black water in previous systems is mixed and used.

29/04/2022 – 31/05/2022

Observations of COD

Without bubbler
(Water hyacinth)
Without bubbler
(Indian shot)
With bubbler
(Water hyacinth)
With bubbler
(Indian shot)
Flood and drain
(Water hyacinth)
Flood and drain
(Indian shot)
Initial COD (mg/lit)
Table no.4 COD readings of 2nd trial

For 3rd trial, black water is collected again and distributed in system on 10/05/2022. The COD of this water is 2244 mg/lit.

Without bubbler
(Water hyacinth)
Without bubbler
(Canna Indica)
With bubbler
(Water hyacinth)
With bubbler
(Canna Indica)
Flood and drain
(Water hyacinth)
Flood and drain
(Canna Indica)
Initial COD (mg/lit)
Table no.5 COD readings of 3rd trial

From the observations of trial taken, it is seen that the sustainability of water hyacinth is less in black water. It starts to wilt in 4-5 days. So in 4th trial, it is decided to dilute the black water with tap water. For dilution following calculation is done:

Fig. 17 Calculation for dilution
Without bubbler
(Water hyacinth)
Without bubbler
(Canna Indica)
With bubbler
(Water hyacinth)
With bubbler
(Canna Indica)
Flood and drain
(Water hyacinth)
Flood and drain
(Water hyacinth)
Table no. 6 COD readings of 4th trial
Fig 18 Plants in 1) Flood and drain & 2) with bubbling and 3)without bubbling


Without bubbler
(Water Hyacinth)
Without bubbler
(Canna Indica)
With bubbler
(Water Hyacinth)
With bubbler
(Canna Indica)
Flood and drain
(Water Hyacinth)
Flood and drain
(Canna Indica)
% losses in COD IN 15 days20%50%59%72%77%90%
Table no. 7


  • COD is decreased in every system. But maximum decrease in COD is found in flood and drain system compared to others.
  • The system with Canna Indica plants shows better results. The COD reduces more in system with Canna Indica.
  • In black water, water hyacinth was sustaining in starting period but after 4-5 days it started to wilt which can be seen from the figure 18. The growth of Canna Indica is better than water hyacinth. The roots were growing well.