Hi all,
I would like to share with you this OPEN ACCESS paper, recently published in Water Science and Technology. I hope it can be useful to some of you, as a simple visual solution for making inferences about the performance of a treatment plant.
Best regards,
Marcos

A simple field essay for detecting departures from expected performance in small-scale, remote or rural wastewater treatment plants
VON SPERLING, M., LIMA, E.M.M.N., MORAES, M.A.A. (2020). A simple field essay for
detecting departures from expected performance in small-scale, remote or rural wastewater
treatment plants. Water Scienceand Technology, 82(7), pp. 1380-1392. doi: 10.2166/wst.2020.421. Abstract

A scientific basis is given to the traditional method of inferring effluent quality based on visualization of samples in transparent
flasks. A scale of 1 to 6, with different printed grey intensities, is placed
behind transparent PET bottles containing the sample, and gives an indication
of the range of turbidity in the sample (1 is the most transparent and can only
be visualized if the effluent is well clarified; in the other spectrum, 6 is
the darkest and indicates highly turbid effluents). Turbidity has been
correlated with total suspended solids (TSS), particulate biochemical oxygen
demand (BOD) and particulate chemical oxygen demand (COD) based on thousands of
monitored data collected in the effluent from seven different treatment
processes in Brazil: UASB reactor, trickling filters, activated sludge,
horizontal wetland, vertical wetland, polishing ponds and coarse filter after
pond. The method is simple and instantaneous, can be used in virtually all
places and in every visit of the operator to the remote treatment plant, allows
recording of the image in smartphones, does not use any equipment, chemicals or
energy, and showed to represent well the effluent quality of existing treatment
plants. This essay is complementary and does not substitute specific
traditional sampling and analysis, but allows easy inference of deterioration
of effluent quality.  

Link ford ownloading (OPEN ACCESS): iwaponline.com/wst/article/82/7/1380/765...ting-departures-from