The descriptions below are process & technology Information Communication output, designed to inform personnel with responsibilities operating, optimizing and managing wastewater treatment processes. The bulletins are short communications authored to address the use of different measurements when working with the technology

A chemical oxygen demand test is done by mixing a sample containing reduced organic carbon with oxidation chemicals, heating it at 150 degrees centigrade to allow for complete oxidation. One of the reaction products changes the sample colour, which can then be measured with a UV-VIS spectrophotometer. The value for COD is calculated from the absorbance data.

When you’ve taken a wastewater sample, a COD measurement can be done from the whole dispersed sample, with the result being an equivalent mass of oxygen of all four the organic fractions in the sample. The fractions are, biodegradable soluble, inert suspended, biodegradable settleable, and inert settleable.

Letting the sample settle, a COD measurement of the supernatant will give the sum of the fractions, biodegradable soluble and inert suspended, which may be a more accurate estimate of the BOD equivalent expected from a BOD five analysis.

When you take a wastewater sample at the start of the process, point 1 on the diagram, the sample contain all four the organic fractions of pollutants as described previously. During the aeration phase, all the pollutants come in contact with the biomass culture and are transferred to the biomass. The fractions of solids in the wastewater transfer to the biomass with different efficiencies, and the COD measurement of the final effluent will give a good estimate of the overall performance of the process, removing pollutants.

Using Oxygen Demand to manage ASP settings and bio-culture acclimatization.

Oxygen demand in a wastewater treatment system describes the measurement of dissolved oxygen uptake by the concentrated bio culture from the water, to use as electron acceptors during metabolic processes. The measurements can be made with normal dissolved oxygen sensors, and the BOD value recorded after a time period, is called the equivalent mass of oxygen, need to convert reduced carbon to carbon dioxide, and reduced nitrogen to nitrate. In simple terms this means dissolved oxygen uptake is measured as a mass, to establish the equivalent mass of biodegradable molecules, like fatty acids and ammonium, that is being consumed by the bio culture.

BOD is measured by measuring the start DO in a sample with a known content of bioculture. After a period of time, the end DO is measured. The difference in DO, start value, minus end value, is the BOD of that sample for the time it was measured in. The three graph plots shown here were created using data measured every six hours, for samples S1, S2 and S3, all with the same culture dilution, but different levels of biodegradable carbon added. Meaningful measurements for BOD may be taken at any point along the curve, and allowing the test to progress until oxygen uptake terminates give the value of ultimate BOD.

The special case of BOD five, a BOD measurement performed to an international standard, ISO 5815-1, is a widely used BOD measurement to measure biodegradable carbon pollutants in water. The measurement is completed in 5 days and the standard describe performing the test to very strict guidelines for dilution, culture seed, temperature and attenuation of unwanted reactions, such as DO consumption from ammonium. cBOD5 is the measurement included and measured for in environmental permits for discharge regulation.

This is data recorded for a BOD test, showing the recorded DO values and calculations to develop usable parameters for process modelling. The data was processed with excel, and the formula for the fitted curve provides an easy way to model the BOD data from an earlier point on the calibrated curve.

Wastewater Treatment influent composition, measuring the different constituents and considering their fate and method of removal, by an activated sludge system. In this section, we also look at the effective mass measurements we may use to determine the pollution load on the treatment system.

Let us consider a sample collected from the crude influent or settled crude, from a position after the primary treatment processes. The sample contains pollutants that include silica, sand, food particles, faeces, toilet paper, plant and animal waste, and organic molecules from medicines, food additives and industrial waste. All the constituents are dispersed through the sample volume, and are either dissolved, suspended or colloidal suspended.  The wastewater pollutant load may be measured at this point with tests like, total solids, total suspended solids, chemical oxygen demand, COD, biochemical oxygen demand, BOD, Total organic carbon, TOC, or total volatile solids. Each of these measurements may give us a clue as to the overall composition of the sample pollution load, and how the activated sludge system is going to respond to the different fractions of pollutants.

If a dispersed sample, shown at one, is left to stand, it settles into two distinct fractions which we mark as the settled fraction in the bottom of the volume, and the supernatant in the top of the volume. The volume-to-volume ratio of the settled sample provide us with another quantification value, the settled volume ration, shown as V, settled, to V, supernatant. The settled fraction of the sample contains most of the particulate constituents, which may be bio-degradable organic, inert organic, which are organic constituents that are not bio-degradable by the activated sludge process, and inert mineral, all of which contribute to the total mass measurements. The supernatant fraction of the sample contains the same three discernible fractions of constituents that are dissolved, small suspended particulate that do not settle in the time of the test, and colloidally suspended undissolved constituents.

Let’s summarise the discernible fractions that we are considering. On the left we can see the totally dispersed sample that contain all the fractions. In the middle we can see the sample settled, with the supernatant volume at the top. On the right the coloured bands represent the volume fractions, determinable by mass measurements of the different analysis methods we have discussed. For reference the fractions are marked with the acronyms corresponding to the coloured band as displayed.

A total solids test is done by evaporating a sample of wastewater containing all the constituents of wastewater, at 105 degrees centigrade. The residue left over after all the water evaporated represents the total solids in the volume of sample used. The total solids measurement includes all the discernible fractions from a wastewater sample, dissolved fractions, suspended and colloidal fractions, and settleable solids of organic and mineral substances.

When you take a wastewater sample at the start of the process, point 1 on the diagram, the sample contain all the solids fractions of constituents as described previously. During the process aeration phase, all the solids in the wastewater stream, come in contact with the biomass culture and are transferred to the biomass. The fractions of solids in the wastewater transfer to the biomass with different efficiencies, and the total solids measurement of the final effluent, will give a good estimate of the overall performance of the process, removing the waste and pollutants.

Total solids measurements are performed by placing a sample of wastewater in an evaporation dish, step 4, and using a laboratory oven to evaporate the water, step 5, until only the dry solids residue remain. The total solids measurement is a subtraction value of the two masses recorded of the evaporation dish, steps 2 and 7.

To evaluate the performance of the process, a total solids measurement of the effluent sample is deducted from a total solids measurement of the influent. The difference represents the total amount of solids that have been transferred to the biomass culture. The mass transfer takes place in the hydraulic retention time of the system. The only mass not measured by total solids measurements, are the mass loss through the process water surface, as gasses.

A suspended solids measurement is done by filtering a sample of wastewater containing the filterable constituents of wastewater through an appropriate filter for the application. The filter is weighed before, slide 2, after drying and acclimatising the filter in a desiccator. The sample size is selected based on the solids load in the sample, slide 3. The sample is filtered with the available filtering apparatus, and placed in the 105 degrees centigrade, pre-heated oven for 2 hours, slide 4 and 5. When all the water has been evaporated from the solid’s residue and filter, the dried filter is weighed again to record the dry solids filterable mass, slide 7. The difference in recorded mass for the filter measurements, is the dry solids for the volume of sample used, which must be converted to a milligram per Liter value with the calculations shown, slide 8 and 9.

The suspended solids value of a sample of mixed liquor is measured in exactly the same way as shown before, with the customary change of using a smaller sample size, due to the high solids in the sample. The MLSS value is crucial to keep track of the accumulated solids of the biomass culture, suspended in the bioreactor. The mixed liquor in the reactor, is a mixture of bioculture floc, complexed and infused by the mass of pollutant solids that are continuously mixed into the reactor. The transfer of the solids in the mixed-in wastewater, to the bio-culture floc in the reactor, describe the filtering action of the reactor as the method of pollutant removal.

A suspended solids analysis may give performance indicators for areas of the process, like the PST, the secondary treatment, and the final effluent quality. Measuring the supernatant of settled samples, gives valuable information about sludge quality, or the settling efficiency if the settling was chemically assisted. Suspended solids analysis is also very accurate to assess jar tests, when determining the dosing rate and concentration efficiency of coagulants.