Dosing Pump capacity

Design Philosophy

(Source: CPHEEO Manual on Water Supply and Treatment, 2013)

The selection of dosing pumps for coagulant chemicals — Aluminum sulphate (alum), poly-Aluminum chloride (PAC), and associated alkali supplements (lime/caustic) — is an integral part of the water treatment plant mechanical design. The CPHEEO Manual on Water Supply recommends that dosing systems be designed for the maximum anticipated chemical dose with adequate safety margins, and that at least one working and one standby pump be provided for each chemical (1W + 1S configuration) to ensure continuity of treatment.

Coagulant Dosing Rate Calculation

The daily chemical requirement is computed from the design flow and the design dose:

The volume of chemical solution to be pumped per hour, at the working solution strength, is:

Solution Strength

Alum and PAC solutions are typically prepared at a working strength of 5–10% w/v (50–100 g/L) as recommended by the CPHEEO Manual, to ensure flowability and prevent crystallization in the dosing lines. The default value used in this calculator is 10% w/v (100 g/L) for alum/PAC. For lime, a lower solution strength of 5% w/v (50 g/L) is adopted due to its lower solubility.

Safety Margin and Pump Sizing

A 10% margin is applied to the calculated dosing rate to arrive at the rated pump capacity, in accordance with good engineering practice. The selected pump shall be capable of delivering this rated capacity at the operating discharge pressure, which is typically 1.5 to 3.0 bar depending on the flash mixer or point-of-injection conditions.

Reference for Dose Selection

The optimum alum or PAC dose for a given raw water quality is determined from jar test results as recommended by the CPHEEO Manual. In the absence of jar test data, the CPHEEO Manual suggests a typical alum dose range of 10–50 mg/L for surface water sources in India, depending on turbidity and color. The Amirtharajah and Mills (1982) coagulation diagram — referenced in Water Quality Engineering by Mark Benjamin — may also be used to select the initial dose for preliminary design.

Calculator

(Note – all Yellow are user defined value, all white are fixed values, all green are calculated value, all orange is calculated not to be shown)

Instructions

Enter values highlighted in the Yellow

Green highlights are the results

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