Lec_7.pptx

environmental engg.

1. In the name of ALLAH, the most Gracious, the most Merciful

2. Environmental Engineering - II Pre-Treatment of Water

3. Course Outline Introduction to Env Engg - II Humans and Environment Selection of the Treatment Processes Water Treatment Processes Wastewater Treatment Objectives of Wastewater Treatment Design of Pre-Requisite Units (sanitary sewer) Pre-Treatment of Water (screening, neutralization / pH adjustment, aeration, chemical pre-treatment, grit chamber, comminutors, and skimming tank) Primary Treatment (coagulation and flocculation, lime-Soda water softening process, sedimentation, solids-contact units) Secondary Treatment of Water (Filtration) Concept of Biological Wastewater Treatment of Wastewater Classification of Biological Wastewater Treatment Processes Design of UASB Reactor (Anaerobic Digestion) Design of Trickling Filter (or Percolating Filter) Design of Oxidation Ponds (or Waste Stabilization Ponds) Sludge Treatment and Disposal Design of an Activated Sludge Process Environmental Impact Assessment Environmental impact of urban and industrial development

4. Includes: Screening Neutralization / pH adjustment Aeration Chemical pre-treatment (self-study) Comminutors Grit chamber Skimming tank Pre-Treatment of Water

5. Skimming tank is provided mainly to remove oil and grease etc. It is generally located before primary sedimentation tank to protect pumps and downstream treatment components. It is so designed that lighter materials like oil and grease content of wastewater rise to the surface of wastewater and remain on top of wastewater until removed. The treated wastewater flows out through the outlet provided below the water line. Skimming Tank

6. Pre-aeration is required to improve its efficiency. Aeration separate grease and oil from the wastewater and wastewater solids and to carry them to the surface . By the addition of air, aerobic conditions are also restored in septic wastewater to improve subsequent treatment Regular cleaning of traps is essential to prevent clogging of inlet and outlet pipes They are mostly rectangular in shape, with hydraulic retention varying from 3 to 30min. Normally, the surface area of 250m 2 is taken for the wastewater flowing at the rate of 1.00m 3 /sec. Skimming Tank

7. Skimming Tank

8. Skimming Tank

9. HDT: 3.0-30min (commonly 10-15min) Surface Area, A s = 250m 2 for 1.0m 3 /sec of wastewater flowing Rectangular: L:W :: 1.5-2.0:1 Height: 3-5m Additional depth of 10-15% for freeboard Design of Skimming Tank

10. Using suitable design criteria, design a skimming tank for a given flow rate of 0.56m 3 /sec Solution: Assume a detention time, t D = 5min = 300sec Volume of the tank, V = Q x t D = 0.56 x 300 = 168m 3 Since, the surface area required for 1.0m 3 /sec = 250m 2 Therefore, for 0.56m 3 /sec, the surface area, A s = 250 x 0.56 = 140m 2 Assuming L:W::1.5:1, W = 10 m, and L =15 m, Thus, actual A S = 10 x 15 = 150m 2 And the depth of tank, D = V/ A s = 168/150 = 1.12m Or the total depth of tank including free board, D = 1.5m Problem (Design of Skimming Tank)