How is the amount of load being applied from the digester to the headworks measured?

The correct choice is the total solids test because it measures the total concentration of solids present in the digester output, which is crucial for understanding the load being applied to the headworks. This measurement is essential as it provides an overall quantity of both suspended and dissolved solids, which directly impacts the treatment processes at the headworks.

Total solids help indicate the strength and volume of the waste being treated, which is vital for designing and operating the treatment system effectively. In a wastewater treatment context, knowing the total solids allows operators to accurately assess the flow and composition of wastewater as it enters the system, ensuring that the treatment process can be managed optimally.

Looking at other options, while BOD (Biochemical Oxygen Demand) and COD (Chemical Oxygen Demand) are important parameters for evaluating the biodegradable organic content and chemical oxygen requirements of the effluent, they do not provide a direct measure of the total solids present. These metrics focus on the organic load rather than the complete estimation of solid content, which is crucial for the loading calculations at the headworks. DPD (N,N-diethyl-p-phenylenediamine) is a method primarily used for measuring chlorine levels and not for solid content in wastewater. Therefore, total solids is the most relevant measure