FISH

Question 1

BOD

Answer 1

• Biochemical demand of oxygen after 5 days
- Only biologically degradable organic substances are measured
- Oxygen amount is assessed in time 0, then we put the water sample into termoregulator
for five days keeping it at dark in 20°C
- After that we measure oxygen concentration again
- The concentration of used/spent oxygen shouldn’t be more than 5 mg/l in Salmonids and
more than 8-15 mg/l in Cyprinids
- Biochemical demand is always lower than chemical demand

Question 2

COD

Answer 2

• Assessed by MnO4- or Cr2O7-
• Non-direct method of assessing oxygen in water
• We detect the value of organic compounds in water - these are reduced by above mention
  ions and for this reduction they need oxygen which they take/spend from water
• Measures both substances biologically degradable and non-degradable
• Optimum is less than 10 mg/l in Salmonids and less than 20 – 30 mg/l in Cyprinids