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.

For the supernatant fraction, the most valuable data would be the Biochemical Oxygen Demand. The dissolved fraction of BOD is by far the most important load fraction for the activated sludge process, as most all the immediate nutrients for the bio-culture to respire with, are carried as dissolved bio-degradable organic molecules. These nutrients would be reduced carbon as sugars, carbohydrates, short, medium and long chain fatty acids, amino acids and proteins. Reduced nitrogen compounds are also a food nutrient for the bio-culture as ammonium and urea are present often in high quantities of above 50mg/L. Micro nutrients as magnesium, calcium, potassium and bicarbonate are also in the dissolved fraction, but considered as the inert mineral constituents vital for bio-culture health. Of the inert mineral micro nutrients, measuring for alkalinity should also be done regularly, or online monitoring if the process is located in a soft water catchment.

Looking at the dissolved biodegradable substance load in the previous section, it is obvious that some common oversights from the last century, are still with us today. Contrary to what are still being taught in wastewater texts and academic courses, the rates of diffusion of dissolved substances, food nutrients and micro nutrients have a significant impact on activated sludge bio-culture health, and the systems performance as a whole. Understanding the model for Fick’s second law of diffusion goes a long way in understanding that the dissolve biodegradable fraction that carry the bio-culture nutrients, diffuse at a much faster rate than any of the other constituents. The reason for this is that the nutrients are being consumed and converted along the pathway of dX. Refer the model on the diagram. This means that the second derivative against time, dT needs to be considered. The concentration gradient for the biodegradable organic nutrients increases dramatically, and hence their rate of diffusion to the internal metabolic centers inside the bio-culture. The significance of this accentuates the importance of using a process indicator, such as the B O D measurement for the supernatant fraction, as a measure for the F to M ratio.

With regards the constituents in the settled fraction, it is not a feasible process to try and isolate the fraction and analyse the content with the measurement techniques we have discussed. Using subtraction from measurements made on sample 1 and 3 is far easier. It is however important to reason through the contribution to the bio-culture made by the particulate biodegradable nutrients in the settled fraction. The quantity of this fraction may be considerable, but there is no way of determining the mass of it, or the biodegradability of it, with any of the measurement techniques at our disposal. Particulate biodegradable organic substances are more complex than the soluble molecules, and it requires much longer residence times, in concentrated bio-culture, for these particles to be enzymatically degraded to simple molecules. This process does not take place in the diluted sample volume of an BOD analysis, and the complex nutrients therefore do not contribute to the oxygen demand to any recordable degree.

The Sludge Volume Index of the mixed liquor sample is measured in millilitre per gram of dry sludge solids. SVI is a critical measurement for routine process assessments and final settlement, mass flux management and optimisation. SVI can effectively be used to determine sludge health, sludge settleability, FST performance, FST mass flux, and RAS concentration.

A sample of Mixed Liquor is collected and placed in a graduated cylinder or SVI measurement device. The sample is left to stand for a specific length of time, and separate into a sludge volume and clear supernatant volume. The measurement can be read of the cylinder graduation as a ratio of Sludge Volume per total Sample Volume. The SVI can be calculated using the volume ratio values, and the Mixed Liquor suspended solids concentration.

The formula to calculate the SVI can be derived from the units of measure, being millilitre per gram. mL/g can be found by taking the settled volume Vs/Total MLSS in the original sample, in gram. (3.5g/L for MLSS at 3500 mg/L and 3.5X5 if 5L were used) The guide for good settleability, using the sludge volume index range of 80 to 120, apply for a mixed liquor solids concentration of 3500 milligram per litre. If your process is operated at a different MLSS concentration, a normalisation calculation may be applied for more predictive results.

A standard linear normalisation to 3500 mg/L can be performed by using this calculation:     

[3500(mg/L) / actual MLSS in mg/L] X calculated SVI

For a MLSS of 2500, the normalised SVI is 1.4 X calculated SVI (110 normalises to 154)

For a MLSS of 4500, the normalised SVI is 0.77 X calculated SVI (110 normalises to 85.5)

It is important to recognise the importance of applying a normalisation factor to the calculated SVI, in order to eliminate the oversite caused by hindered settling effects during the measurements. On the graphic below, (experimentally determined settling densities of the same fresh sludge sample) we can see that the settled volume (Z), of a sample change, not only with a change in MLSS concentration, but also because the settling density of the sample becomes less, with an increase in MLSS concentration. (X)

The variation in settling density between different concentrations of sludge with the same settleability (K), is described with the Vesilind equation, often used when assessing the settling performance in an FST, at different concentrations. The normalisation above is linear, whereas the variance from the Vesilind equation is exponential, but the slope of the Vesilind equation where hindered settling dominate, is near linear with an R2 often near 1.

Process respirometry analysis can be performed manually with standard laboratory analysis equipment, or with an online system plumbed in to deliver instant measured results in real time. Process respirometry measurements are based on Dissolved Oxygen measurements of small samples separated from the reactor's main aeration zone before a measurement is taken. 

Parameters such as Biochemical Oxygen Demand (BOD), Oxygen Uptake Rate (OUR), and Specific Oxygen Uptake Rate (SOUR) can be calculated using the measured dissolved oxygen measurements. Some respirometry systems use calibrated results based on alternative measured parameters, such as COD measurements. 

The calculated BOD, OUR or SOUR values can be used to determine the bio-culture health and performance, the biodegradability of the incoming wastewater, the VFA breakdown and level of pre-acidification of the process feed, to detect potential metabolic suppression of toxicity agents in the influent, and to set up process kinetic parameters and dynamic process management responses around circadian flow cycles or seasonal fluctuations of the wastewater influent. 

The diagram below shows basic components of an online respirometer using an extracted sample from the start of the process (S), an automated separation module (A) splitting the flow into two channels (A, 1, 2), a mixing and chemical addition module (B), the initial DO measurements (C, 1, 2), the flowthrough pathway coils (D, 1, 2), and the end DO measurements (E, 1, 2). 

The collected sample (S) includes the bio-culture (MLSS) and is split by a filtration system (A) into two lines for potential OUR/BOD determinations(A1) and potential SOUR determinations(A2). The module A is fitted with a flush and dilution line (DI water), and the module B is fitted with a dilution line (DI water), and chemical addition or carbon source standard (BOD standards) lines for sample preparation. 

The DO data is recorded for intervals of time (F, usually in minutes), and the data is processed on the SCADA or computer system (G) to produce the BOD, OUR or SOUR values. 

A substantial amount of knowledge and practical analytical experience is required to participate in the management of an ASP process with online respirometry. The experience doing so, however, may turn attending personnel into ultimate specialists for ASP process performance, as the respirometry relates the core concepts of the ASP process to tangible, measurable parameters.