Clay Concentration in Stock Solution for Desired Influent Turbidity

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Abstract

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Introduction

A clay suspension is used to feed the turbid water to the flocculator. During the summer 2007, the flocculator group used a stock suspension of 3,000 NTU. We wanted to reproduce the stock suspension of 3,000 NTU but needed to determine what turbidity would be produced from a given suspension of kaolin clay. The influent turbidity is user defined in Process Controller, and the program will dictate the flow rate of the clay suspension into the setup in order to achieve the influent turbidity.

Arbitrarily adding clay to reach a certain turbidity is ineffective and wastes time. Past research did not include any relationship between the amount of clay added and the respective final turbidity of the stock solution. We designed a simple experiment to determine what amount of kaolin clay should be added per liter to create a desired turbidity.

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Methods

The experiment ran by adding different amounts of kaolin clay (50 mg, 100 mg, 250 mg, 500 mg, and 1000 mg) to 1 L of water and measuring the turbidity of the mixture with the Hach 2100 N table turbidimeter. The amount of clay was weighed on an electric scale. The solution was hand mixed for several minutes before pouring into the glass cuvette tubes. The cuvette tubes were wiped with Kim wipes before being placed in the turbidimeter. Since the meter could not adequately read turbidity readings above 200 NTU, we only tested up to this maximum turbidity value and calculated a regression curve based on the sampled data points using Microsoft Excel. After the relationship was determined, a new stock concentration of 3,000 NTU was made using this regression formula.

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Results

As mentioned in the procedure section, we measured the turbidity of several clay-water mixtures in an effort to determine the relationship between concentration of clay and NTU. The linear regression formula derived from the 5 data points (Figure 1) we measured suggested that at a clay concentration of 1.117 g/L was needed to create a solution of 3000 NTU.

We cleaned out the vial in the influent turbidimeter and input into Process Controller 3000 NTU as the value for the stock concentration. We programmed 100 NTU as the desired influent turbidity into the flocculator and ran the "3-New Flow and Sample" state to test whether we had indeed created a 3000 NTU stock. After some initial fluctuations, the steady state turbidity achieved was around 40 NTU. Our expected value was 100 NTU. This discrepancy may have been caused either by the fact that our clay solution was actually less turbid that our calculations had indicated, or that the program incorrectly calculated the dilution. This problem did not effect our experiment, however, as we corrected the turbidity error by applying the below equation to the observed turbidity readings.

Figure 1: Relationship between amount of clay added (mg/L) and the resulting turbidity (NTU) of the clay stock solution.

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Discussion

This experiment was soley to obtain a linear regression relationship between the amount of clay added by mass and the resulting turbidity in a 1 L solution. This relationship and equation will be used in future experiments in order to determine the amount of clay to be added in order to achieve a certain clay stock solution.
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