Lecture (8): liquid wastes treatment (primary, vital, advanced).

Lecture (8): Topic (1) liquid wastes treatment (primary, vital, advanced). Lecture (7): Lecture objectives: By the end of this lecture students should be able to: To know the conventional sewage treatment plants. To know the selection of specific processes. To know the reusing of wastewater.

Lecture (8): Topic (1) liquid wastes treatment (primary, vital, advanced). Conventional Sewage Treatment Plants: the conventional plant’s performance is usually measured by reductions in suspended matter, BOD, and bacteria. The processes in conventional plants include: 1) pretreatment, 2) settling, 3) chemical treatment, 4) biological oxidation, 5) disinfection, and 6) sludge conditioning and disposal processes

Selection of Specific Processes: In the selection of specific processes and plant design, points must be consider: The existing and potential regulatory standards; Plant operators’ requirements and availability; Existing and projected sewage flow; Flow pattern and waste characteristics; Climate, Topography, Availability of land; Plant location within the community; All aspects of cost.

(1) Pretreatment Processes: Objectives: To protect pumping equipment. Control and monitor instruments. Prevent clogging filters. Pretreatment equipment includes: Screens, grinders, skimmers, and grit chambers. Flow equalization is also a pretreatment process.

(2) Settling or Clarification Remove settleable solids by gravity. A rectangular or square tank . Settling tanks are commonly designed based on the: Overflow rate, The unit volume of flow (gallons per day per square foot). The detention times for settling .

(3) Chemical Treatment: The addition of chemical coagulants is an effective way to improve the settling characteristics of suspended solids. This process may also be a feasible alternative to: Improve the efficiency of existing. and/or overloaded settling basins.

A comparison of advantages and disadvantages of the more common coagulants is presented in table below: Coagulant Advantage Disadvantage Alum Easy to handle and apply; most effective between pH 6.8–7.5 Adds dissolved solids (salts) to water, effective over a limited pH range Ferric chloride Effective between pH 4–11; also makes sludge dewatering, Adds dissolved solids (salts) to water Ferric sulfate Effective between pH 4–6 and 8.8 – 9.2 Adds dissolved solids (salts) to water; usually need to add alkalinity Ferrous sulfate Not as pH-sensitive as lime lime Commonly used, very effective; may not increase TDS; sludge dewaters easily Very pH dependent; produces large quantities of sludges; overdose can result in poor effluent quality Polymer Small dosage usually needed, easy to handle and feed Improper dose results in poor floc formation

(4) Biological treatment: Two basic techniques: (4:A) Fixed-bed: The trickling filter has a fixed-bed of stone or plastic packing material that provides a growth surface for specific bacteria and other organisms. (4:B) Fluid-bed: The activated-sludge processes and sewage lagoons are fluid-bed systems.

(4:A) Trickling Filters Trickling filters are the most common treatment units used by municipalities to provide aerobic biological treatment. Trickling filters are classified according to: The hydraulic. Organic loading applied.

(4:B:1) Activated Sludge Activated-sludge processes use continuous agitation and artificially supplied aeration of settled sewage together with recirculation of a portion of the active sludge that settles in a separate clarifier back to the aeration tanks. These processes vary in: Detention time, Method of mixing and aeration, Technique of introducing the waste, Recirculated sludge.

(4:B:2) Oxidation Ponds Lagoons and ponds have many applications ranging from complete raw waste treatment to polishing a secondary plant’s effluents. The applications do not use metal or concrete tanks. A lagoon is a pond of design that receives waste that has not been settled or exposed to biological oxidation prior to entering it.

(5) Disinfection Conventional treatment plants use chlorination as the final treatment process to reduce bacteria concentration. Disinfection involves the destruction of disease-causing organisms. The most common means of wastewater disinfection is chlorination.

(6) Treatment of drainage sludge The solid material deposited in the settlement tanks is known as sludge. It is: a thickish, offensive smelling, liquid with a water content ranging from 60% to 95%.

(6:1) Thickening Thickening: The first stage in treatment is to remove as much water as possible in order to reduce the volume of material. As the water is removed the solids concentration increases.

(6:2) Dewatering Dewatering This is the process increases the solids concentration up to around 40% are known as dewatering and improvements in dewatering technology has led the production of drier cakes. Dewatering is carried out using either centrifuges, belt presses or plate presses

Enhancement Enhancement processes do more than simply remove water and they fundamentally change the nature of the sludge itself. There are two processes: Anaerobic digestion. Composting.

Topic (3) Reusing wastewater Wastewater reuse can be grouped into the following categories: • Urban reuse—(irrigation of public parks, ……..) • Agricultural reuse—(irrigation of nonfood crops). • Recreational impoundments—such as ponds and lakes. • Environmental reuse— creating artificial wetlands, • Industrial reuse—process or makeup water and cooling tower water.