Test: Environmental Engineering- 5 - Civil Engineering (CE) MCQ

The main advantage of oxidation ditch is the ability to achieve removal performance objective with low operational requirements and eminence costs.An oxidation ditch is a modified activated sludge biological treatment process that uses long solids retention times (SRTs) to remove biodegradable organics. The typical oxidation ditch is equipped with aeration rotors or brushes that provide aeration and circulation. The wastewater moves through the ditch at 1 to 2ft/s. The ditch may be designed for continuous or intermittent operation. Because of this feature, this process may be adaptable to the fluctuations in flows and loadings associated with recreation area wastewater production. Several manufacturers have developed modifications to the oxidation ditch design to remove nutrients in conditions cycled or phased between the anoxic and aerobic states.

Oxidation ponds, also called lagoons or stabilization ponds, are large, shallow ponds designed to treat wastewater through the interaction of sunlight bacteria, and algae. Algae grow using energy from the sun and carbon dioxide and inorganic compounds released by bacteria in water. During the process of photosynthesis, the algae release oxygen needed by aerobic bacteria. Mechanical aerators are sometimes installed to supply yet more oxygen, thereby reducing the required size of the pond. Sludge deposits in the pond must eventually be removed by dredging. Algae remaining in the pond effluent can be removed by filtration or by a combination of chemical treatment and settling.

Anaerobic digestion is normally adopted for primary sludge because primary sludge contains large amount of readily available organics that would induce rapid growth, of biomass if treated anaerobically.

Anaerobic decomposition process produce less biomass and primary function of anaerobic digester is to convert as much of sludge as possible to end products like liquid and gases while producing little biomass.

Waste-water contains wide varieties of organism.

These organisms are broadly classified as

a) Acid formers

b) Methane formers

Acid formers: Acid formers consists of facultative and anaerobic bacteria and include organisms which solubilize the organic acid through hydrolysis.The soluble end products are then fermented to acids and alcohol of lower molecular weight.

Methane formers: Methane formers are strictly anaerobic and converts acid and alcohol alongwith hydrogen and CO₂ to methane.

Concentration of returned sludge =

= 10869.56 mg/l

The average time for which the biomass stays in the system is called sludge age

where V = volume of the tank

X = mixed liquor suspended solids concentration in the aeration tank

Q_w = flow of wastewater

X_u = Biomass leaving the aeration tank per day

⇒ 100 – P = 2 × 2

⇒ P = 100 – 4

⇒ P = 96%

The efficiency of a high rate single stage trickling filter is given as:

Where,

Y = Total organic loading in kg/day

V= Filter volume in hectare. m

F = Recirculation factor = 1.25 (given)

BOD to be removed in filter

= Total BOD of sewage – BOD removed in primary clarifier

= 200 mg/L – 35% of 200 mg/L

= 200 – 70

= 130 mg/L

Y = total organic loading

= 130 mg/L × 10 × 10⁶ L/d

= 1300 kg/d

n = 79.26%

Total BOD produced per day = 
Organic loading rate = 200 kg/day/ha

According to definition

sludge age 

Q_w = 100 m³/day