Water and Waste Water
The table on the next page gives a list of sections which should be included in the report, and a detailed marking scheme for the report. Some sections are not applicable to both types of treatment plant.Under each section, you should describe the background to the process, and give specific details of how it is carried out in the plant you visited. You may mention alternative processes, but there is no need to describe alternatives in detail. If the plant visited does not have a particular process, the reasons and implications of not having it should be discussed.
When you visit the plant, it would be useful to have prepared a list of questions in advance.You should look for reasonably detailed technical data – volumes, water quality, quantities of chemicals used etc.It is also a good idea to bring a camera.
When writing the report you will need to get additional background information from textbooks and web sites.All material taken from these sources should be clearly referenced in the body of the report, with a list of references at the end.Copying material from sources without proper referencing
Metric (preferably SI) Units should be used throughout – for example do not give flows in gallons per day, or tank dimensions in feet.
Detailed Marking Scheme
Section Required details Water Wastewater
Introduction History of plant, background to need for treatment 2.5 2.5
Location map / description Map of area, with plant and intake / outfall. 1.5 1.5
Population Population equivalent, related to volumes 2.5 2.5
Hydraulic Loading Treated volume 1.5 2.5
Organic Loading kg BOD / BOD concentration n/a 2.5
Overall process diagram / description Diagram (60), narrative(40) of flow through plant 5 5
Water quality Water quality standards required 2.5 n/a
Water sources Source, cleanliness, adequacy 1.5 n/a
Effluent Quality Effluent standards n/a 3
Site layout Layout drawing 2.5 2.5
Pre-treatment Screening, grit, FOG removal, storm water – technical details 3.5 5
Coagulation / Flocculation Chemicals, process, tanks sizes 5 n/a
Sedimentation / primary treatment Tank sizes, types, cross-section 6 8
Filtration Filter bed size, depth of layers, cross-section 7 n/a
Disinfection Chemical dosage, methods, variations, residual 5 n/a
Fluoridation Reasons, dosage levels, methods 3 n/a
Softening, pH correction Reasons, dosage levels, methods 2 n/a
Storage and distribution Brief description of reservoirs, distribution network 3 n/a
Secondary Biological treatment Process description, tank sizes, flows, retention times, sludge volumes, RAS % n/a 11
Tertiary treatment – filtration / disinfection Process description n/a 3
Monitoring / Testing / computer systems Description of tests, frequency, actions 3.5 3.5
Sludge treatment Thickening, digestion, dewatering, drying, cross-section of plant 2 5
Sludge disposal Landfill / agricultural land – quality, pollutants, quantity 3 4
Cost analysis Capital & operating costs 2.5 2.5
Odour / noise Odour control, noise control 1.5 3
Chemicals Chemical handling, storage 2.5 2.5
Power consumption Energy management, backup supply 1 1
Health & Safety / Contingency planning Procedures 3 3
Report layout & writing style Visual, spelling & grammar, photos + captions, drawings + context 5 5
Discretionary Overall impression 7 6.5
“Specificity” Is everything related to plant visited? 15 15
Total 100 100
If the background to a process is explained well, but no link to specific plant, 50%
Specific plant description, but little or no explanation of process 60%
Background to process plus specific plant description 70-100%
Coagulation / flocculation
Coagulation is an essential process in the treatment of water for drinking and also wastewater treatment. Metal sulfates were used as coagulants, in particular Aluminium sulfate. This was done in order to remove the smaller suspended particles from the water. The water is pumped through a stored in a 1300 m3 tank (McCarthy Hyder Consultants, 2006).
The pre-treated sewage is held temporarily in a tank in order to allow heavy solids to settle at the bottom and lighter solids together with oil and grease to rise to the top. Once the sunken solids and the floating impurities have been removed, the remaining water may be discharged or subjected to further treatment known as secondary treatment. In order to screen the water, the plant is fed from 1m wide channels. Four removal lines are used to remove sand and grease from the water that has undergone screening. Each removal line has a width of 6m and is 25m long. The screened water is then transferred to a storm tank of size 1300 m3 and an automatic penstock cuts it from the inlet chamber. The penstock opens automatically to divert waste water when the inlet chamber gets empty (McCarthy Hyder Consultants, 2006).
Storage and distribution
Seven pumps complete the raw sewage lifting and pumping system. Three of these pumps are rated at 55kW while the other 4 are rated at 200kW. At any time during operation, six of the pumps are used while the 7th one is held as a redundant pump in case of failure. The biological reactor id the plant is composed of four streams. Each stream is subdivided into 600 m3 capacity and 3000 m3 capacity for anaerobic and aerobic zones respectively. An odour control system covers the entire plant, with an odour treatment system installed at each step of the whole water treatment process. In order to remedy the flaws found during the commissioning of the plant, additional air treatment units were installed (McCarthy Hyder Consultants, 2006).
Thickening is the first process in sludge treatment and occurs in a tank known as a gravity thickener. The volume of the sludge is reduced to less than half. After thickening, digestion takes place. The plant makes use of anaerobic digestion whereby organic solids are broken down into stable substances. The sludge is then dewatered through a combination of evaporation and draining through the sand via gravity. After dewatering the sludge undergoes thermal drying to remove all the water and have the solid sludge ready for disposal (Nathanson, 2018).
The transportation and disposal of sludge is a significant part of sludge management especially when considering the impacts on the environment and the cost. Sludge from Shanganagh Bray WwTW (Wastewater Treatment Works) is buried in sanitized underground landfills since Sludge contains fertilizer and is a soil conditioner. However, due to the presence of toxic chemicals, the land is not available for agricultural use for crops meant for human consumption (Nathanson, 2018).
McCarthy Hyder Consultants. (2006). Shanganagh & Bray Main Drainage Scheme. Employer’s Requirements. Bray: Environmental Protection Agency (EPA).
Nathanson, J. (2018, April 05). Wastewater Treatment. Retrieved from Encyclopaedia Brittanica Inc: https://www.britannica.com/technology/wastewater-treatment/Sludge-treatment-and-disposal