Primary clarifiers

Points of Chemical Addition In independent physical-chemical treatment or in phosphate removal in the primary clarifier ahead of biological treatment, chemicals are added to raw sewage. In tertiary treatment for phosphate removal and suspended solids (SS) reduction, they are added to secondary effluent. In both cases, proper mixing and flocculation units are needed. For phosphate removal or improvement of SS capmre in biological secondary treatment, chemicals are often added directly to aeration units or prior to secondary settling units, without separate mixing and flocculation. In some phosphate removal applications coagulants are added at... 

In a typical plant operation the pollutants dissolved in the wastewater or that would not settle in the primary clarifiers flow on in the wastewater to the Secondary treatment process. Secondary treatment further reduces organic matter (BOD,) through the addition of oxygen to the wastewater which provides an aerobic environment for microorganisms to biologically break down this remaining organic matter. 

Ravazzini AM, Nieuwenhuijzen AF, van der Graaf JHMJ (2005) Direct ultrafiltration of municipal wastewater comparison between filtration of raw sewage and primary clarifier effluent. Desalination 178 51-62... 

An air monitoring study at a waste-water treatment plant in Australia found time-averaged hydrogen sulfide levels of 1-2 ppm near the primary clarifiers and inlet structure, and levels <1 ppm at various other locations in the 10-hectare plant site (Koe 1985). 

Some fixed filtration systems are relatively inexpensive. For example, constructed wetlands and sand filters are generally the most successful methods of polishing the treated wastewater effluent from lagoons. These systems have also been used with more traditional, engineered primary treatment technologies such as septic tanks and primary clarifiers. In such constructed wetlands, the system utilizes the roots of plants to provide substrate for the growth of attached bacteria, which utilize the nutrients present in the effluents and for the transfer of oxygen. 

In recent years, environmental concerns over water quality have led to installation of waste water treatment plants at pulp and paper mills. The underflow from the primary clarifier has generated another solid waste disposal problem. To solve this problem, some mills are feeding clarifier sludge to hog fuel boilers. The high moisture content of the sludge in conjunction with its low Btu content creates more difficult operating conditions for the furnace. Comparative composition of TDF, coal, wood waste,... 

Duff SJ, Moritz JW, Andersen KL. Simultaneous hydrolysis and fermentation of pulp mill primary clarifier sludge. Can J Chem Eng 1994 72 1013-1020. 

Example 15.5 A domestic wastewater with a flow of 20,000 m /d is to be nitrified. After settling in the primary clarifier, its BOD5 was reduced to 150 mg/L. The concentration of nitrate nitrogen after nitrification is 30 mg/L. (a) Calculate the amount of bicarbonate alkalinity required for nitrification, (b) Determine if the primary treated sewage can produce enough alkalinity to satisfy nitrification requirement. Assume that 91% of the nitrate nitrogen is destroyed in denitrification. 

Qo = influent flow to the process from the effluent of the primary clarifier So = substrate remaining after settling in the primary clarifier and introduced into the reactor... 

Influent flow to the process from the effluent of the primary clarifier... 

Substrate remaining after settling in the primary clarifier, mg/L... 

Mass loading rate of solids to the primary clarifier (at a flow of 30 million gallons per day (MOD) and a suspended solids concentration of 290 mg/L). 

Mass production rate of solids in raw sludge (primary clarifier underflow) for 60% removal. 

Hydraulic flow (underflow) of raw sludge from primary clarifier (1 lbH2O/1.051b raw sludge 6.51b dry solids/100 lb raw sludge). 

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