PRIMARY TREATMENT/PRIMARY CLARIFIER FOR WASTEWATER TREATMENT

PRIMARY TREATMENT/PRIMARY CLARIFIER FOR WASTEWATER TREATMENT

Screening

There are large grits and solids contained in wastewater that can interfere with processes of treatment or lead to unwarranted mechanical wear and tear as well as the increased cost of maintenance on equipment of wastewater treatment. These materials require separate handling to minimize potential problems. The removal of these constituents from the influent wastewater requires preliminary treatment, such as flow equalization, odour control, septage handling, grit removal and screening. This section only focuses on screening, which is the first operation unit employed at WWTPs (wastewater treatment plants). At the screening unit, objects such as metals, plastics, paper, and rags are removed to prevent clogging and damaging of appurtenances, piping, and equipment at the downstream. There two types of screening that is used in most contemporary wastewater treatment plants, namely fine screens and coarse screens; some use both. Coarse Screens is used to remove debris, rags, and large solids from wastewater. Usually, it has openings which are 6 millimetres (0.25 inches) or larger. There are three types of coarse screens, namely the trash racks, manually and mechanically cleaned coarse/bar screens. Trash racks have an opening size of 38mm to 150 mm, that of manually cleaned bar screen is between 30mm to 50mm with the bars set vertically at 30^o-45^o for facilitating cleaning and mechanically cleaned bar screen being 6mm to 38mm (Qasim, 2017). Fine Screens is a type of screen that is used to remove materials with the potential of creating challenges of operation and maintenance in downstream processes, especially in systems without primary treatment. Typically, fine screens have opening sizes of 1.5mm to 6mm while opening sizes for very fine screens is between 0.2mm and 1.5 mm. Fine screens are used to reduce levels of suspended solids the same as those achieved through primary clarification.

Design Calculations

1. Selection of Sludge age () (Takács et al., 2008)

Where

f_b is biodegradable fraction of volatile suspended solids = 0.63

K_d is the endogenous decay coefficient = 0.07 per day

The assumption made here is that the   value will be checked later.

1. Calculating Effluent Soluble BOD₅ and Efficiency

Now, set  to be 20 days then the effluent soluble BOD₅ and efficiency is calculated using the equation;

BOD of VSS in effluent at day five is given by

But VSS/SS = 0.7,

The total BOD effluent at day five is given by effluent soluble BOD₅ plus BOD of VSS in effluent at day 5;

The calculated total effluent BOD₅ is 14.08 mg/l, which is within 20 mg/l, hence it is acceptable (Van Haandel and Van Der Lubbe, 2007). Given that the influent BOD₅ is equals to 370 mg/l, then the efficiency of the soluble BOD₅ removed is calculated using the equation;

The overall efficiency of BOD₅ and efficiency removal is calculated as;

• Aeration Lagoon Design Calculations

In this calculation, two assumptions are made; first, the Mixed Liquor Suspended Solids (MLSS) is 4000 mg/l while the Mixed Liquor Volatile Suspended Solids is (4000*0.7) = 2800 mg/l. The second assumption is that the solids in the final settling tank are neglected; this is expressed using the equation below;

Given that

The volume of aeration tank is approximately 32,836 cubic meters. However, the dimension of the aeration lagoon measuring 2.5 m by 30 m by 76m; thus, the volume is;

Therefore, the number of aeration lagoons;

It would require 6 aeration lagoons, 3 working plus 3 for future. The word future is used to refer to 12 years from the time when the projected was executed.

1. Detention Time, t, calculation

Time of detention, t, is given as;

Where,  is the aeration tank volume and Q_a is the flow rate

The Food to Microorganism ratio (F/M), (kg BOD₅ /kg MLVSS-day) is checked as;

1. Return Sludge Pumping

Here, it is assumed that the return one percent of the sludge is solids, therefore,

Where, Xu is the underflow from clarifier that contains settled solids; given that mixed liquor suspended solids (MLSS) is 4000mg/l, the recycle ratio would be calculated as;

1. Surplus production of sludge

Net Volatile Suspended Solids (VSS) produced per day is calculated as;

In reference to SS, the net production is given by dividing net VSS produced per day by endogenous decay coefficient:

Assuming that the suspended solids (SS) are removed as underflow from the final settling tank with 1% solids; given M_s = 6567.2 kg/day, = 1000 kg/m3, S_s=1.03 and P_s = 1% or 0.01, then the volume of sludge is calculated as;

This is the sludge volume to be withdrawn every day to sludge drying beds.

• Biodegradable VSS fraction

Therefore, fb

• Removing Phosphorus

The amount of phosphorus removed every day is approximately 106 kilogram

1. Removing Nitrogen

The amount of nitrogen removed every single day is approximately 510 kilogram

1. Oxygen Demand

The amount of oxygen needed to meet carbonaceous as;

Converting from BOD5 to BODu

The amount of Oxygen required to obtain nitrification assuming nitrogen lost in denitrification, effluent and nitrogen removed in sludge as a maximum upper limit of 4.33 (influent TKN per day) (Van Haandel and Van Der Lubbe, 2007)

1. Required Power

It is assumed that the aerators efficiency transfer = (2 kgO₂ /kw.hr) at standard conditions, and 70% capacity at field conditions (Austin, D. and Nivala, 2009).

But there are 6 lagoons, therefore, power require for each lagoon is calculated as;

Also, it is important to note that each lagoon has 3 surface aerators service, thus, the power required for each service is;

Hence, 50 kw is the design power that the manufacturer has to meet.

Which is acceptable since it is within the range of 8 to 16 hence, the settling based on the extended aeration; decision, SOR acceptable.

References

Austin, D. and Nivala, J., 2009. Energy requirements for nitrification and biological nitrogen removal in engineered wetlands. Ecological engineering, 35(2), pp.184-192.

Parker, H.W., 1975. Wastewater systems engineering. Prentice Hall.

Qasim, S.R., 2017. Wastewater treatment plants: planning, design, and operation. Routledge.

Takács, I., Stricker, A.E., Achleitner, S., Barrie, A., Rauch, W. and Murthy, S., 2008. Do you know your sludge age? Proceedings of the Water Environment Federation, 2008(13), pp.3639-3655.

Van Haandel, A. and Van Der Lubbe, J., 2007. Handbook biological waste water treatment-design and optimisation of activated sludge systems. Webshop Wastewater Handbook.