Turbidity-based monitoring of particle concentrations and flocculant requirement in wastewater pre-treatment

The removal of particulate organic material in the first step of wastewater treatment may result in significant savings of reactor volume and energy at wastewater treatment plants, because the organic loading to pursuing unit operations can be reduced. This article describes experiments into the possibility of using turbidity measurements as a tool to quantify the concentration of particles in raw wastewater and, based on the result, assess the organic polymer requirement. A feed forward polymer dosing strategy based on a fixed polymer to influent-turbidity ratio was developed and tested. The experimental work confirms that turbidity measurements can be used to quantify particulate COD. For the investigated wastewater (both untreated and flocculated samples) a linear relationship was found in a wide range of particulate COD (100–900 mg O₂.l⁻¹) and turbidity (50–450 NTU). On-line turbidity measurements showed that the particle concentrations in the tested municipal wastewater varied significantly. During dry weather conditions the turbidity fluctuated from 100 to 400 NTU, while in rainy periods fluctuations of 100 to >1,000 NTU were measured. The tested turbidity-related polymer dosing method could be used to create different, constant levels of particle removal, despite large particle concentration variations in the influent. Moreover, it resulted in higher removal efficiencies and a more stable operation compared to the dosing of fixed polymer dose per unit of volume.