Antônio, I have found that changes in total BOD and TOC during sewage conveyance were too small to be significant and often contradictory, however, there appeared to be a small decrease in BOD and a definite increase in suspended solids after pumping. 

Primary tank BOD removal efficiencies were also less efficient with older “septic” sewages. 

The main issue is sulphide generation, particularly after pumping through long rising mains and with higher sewage temperatures.  Typical sulphide concentrations can increase from 0.2 to 1.0 or 10 mg/L or more.  Problems caused by high sulphides include odour emission, corrosion of concrete, copper and steel, activated sludge bulking and occupational health risks.

Dear Michael Lever

Thank you for sharing your valuable experience and expertise in this area. In fact, an experienced colleague in wastewater treatment plant design has similar opinions in several aspects that you mentioned. As you pointed out, sometimes it is possible to find small contradictory changes.

Besides the well-known sulfide buildup problem, I would like particularly to discuss what happens exactly with the wastewater characteristics when the detention time is really very, very large in very, very large networks. Also, I would like to hear about the changes that exactly occur in the downstream treatment when air or oxygen is introduced in the network for control of sulfide.

When you write that the BOD and COD increased, how do you estimate that there has been an increase? Compared to what?

There are various models of in-sewer processes, which have been consolidated in an IWA report on the subject. In part the effect will depend upon your temperature and sewer layout. Cold temperatures, steep gradients (hilly country) will minimise transformations; high temperatures, flat sewers, lots of inline storage will increase transformations.

But the transformations should not cause COD to increase. They may cause BOD to increase, from a conversion of slowly degradable COD to rapidly degradable COD (fermentation).

Dear Jeremy

Thanks for your answer and pertinent question. According to the measurements, BOD and COD increased between the wet-well that feeds the pump in the pumping station and the discharge (in the outlet chamber) of the pressurized pipe (force main). Temperatures were between 20 and 25 degrees Celsius.

The BOD increase can potentially be explained by the temperature & forced main creating conditions to promote breakdown of COD into more degradable forms. But the COD increase would suggest that you took a few spot samples and are not looking at like for like; rather that you have different points on a diurnal curve. A less likely problem is that the lab procedures for QA are poor - preservation, homogenisation, accuracy of procedure.

Intorducing air for sulphide control should have a side-effect of a reduction in BOD/COD. The bacteria that use BOD/COD have a faster growth rate than those that oxidise sulphur - the sulphur oxidisers are of the same order, or slower, than nitrifiers.

Dear Jeremy. Thanks for your important contribution. I did not tag along directly the procedure and the analyses, but the Laboratory is relatively reliable. The hypothesis that you suggest makes sense and is perhaps a possibility, but the doubt remains because that happened in two distinct force mains. So, if I understood, at a first insight for you and according to the report you mention, the interference in the treatment is basically in the BOD, or in the BOD and COD, and does not affect perceptively the quality and efficiency of the treatment, and the expected quality of the final effluent, regardless the dimension of the system (even with multiple pumping systems, and pressurised pipes, with or without artificial aeration and hot temperatures).

Antônio, please note results from oxygen injection trials with a rising main diameter 1.37 m, length 9.6 km and average detention time of 2 hours.  Liquid oxygen was injected at an average rate of 30 mg/L.  After the initial O2 demand was satisfied, oxygen was consumed fairly linearly along the main at 7 mg/L.h (23°C) and 12 mg/L.h (28°C) approximately equally by wall slimes and sewage.  The plate count in the sewage increased from 20x10^6 to 25x10^6 with oxygen injection but with no significant reduction in BOD detected (however see papers by Boon and Lister and also Newcombe et. al: Wat. Pollut. Control, Vol 78 No 4, 1979). 

With no oxygen, average 24 h composite BOD values appeared to reduce slightly from 320 in the wet well to 310 mg/L at the delivery point (31 samples, standard deviation 55 mg/L).  60% of samples had a lower BOD after pumping, however, if medians were used, BOD values increased from 295 to 305 mg/L. Mean TOC values increased from 190 to 195 mg/L. 

With oxygen injection, 24 h composite BOD values were unchanged at 325 mg/L with SD inexplicably higher (85 mg/L, 148 samples).  54% of samples had a lower BOD at the delivery point with Geo-mean values reduced from 313 to 311 mg/L but median values increasing from 321 to 325 mg/L.  Mean TOC concentrations were unchanged (210 mg/L). Suspended solids increased from 220 to 255 mg/L and % soluble BOD decreased from 265 to 240 mg/L.   

Bio-flocculation decreased soluble BOD from 81% to 74% resulting in improved BOD removal in primary tanks from 30% to 34% and TOC removal from 30% to 35%.

Oxygenation reducing BOD/COD can be understood. The work of Boon & Lister needs to look at error bands/standard deviations to decide if the increase/decrease is statistically significant. Medians/averages are all affected by the effect of diurnal variation, including variation in flow - a comparison in kg per day, using flow-weighted composites, would be needed. It is not clear if Antonio is comparing spots, time-weighted composites or flow-weighted composites, or how large his detected increase is; but still the discrepancy is most likely to be explained by diurnal variations.

Hi Antonio,

you're faced with a very complex mix of transport, (bio)chemical and physical degredation processes. So by just measuring the biochemical parameters in a stretch of sewer will never give the info to get a good mass balance for all parameters. A few years ago Jaco Huisman published his PhD thesis on this subject, you can find it on researchgate: huisman proefschrift-corr2-3 - ResearchGate.

best regards

Francois Clemens

Thanks Michael, everything becomes simpler and efficacious when we matured our knowledge. Dear Jeremy and Francois, the hypothesis that Jeremy suggested may have some consistency. The measurements in the (5 and 7) samples, in each one of the two points (wet well), were CBO, CQO, Temperature, pH (between about 7 and 8), Sulphate, Dissolved Oxygen and Sulphide (of course that some other parameters could be measured, depending on the problem you intend to study). They were performed for a work under my supervision several years ago, but I did not follow closely the details of the procedures used. The average increase for CBO and CQO, with respect to simple average of the (punctual) samples, was roughly about 20% and 35% (CQO increase slightly higher than CBO increase in the first case and lower in the second). The volume of both force mains was higher than the net volume of the wet well (probably different wastewater in the samples of the two measured points), and the higher increase occurred exactly for the pumping station with the lower flow (which is also on the side of the Jeremy hypothesis, that is, diurnal variations of the wastewater characteristics).

In fact, there are several works and PhD on this subject. Concerning to the main question, Jeremy did not answer to my provocation. The transformations in the network can be relevant in the treatment or not?

Antônio,

I think there are cost and engineering constraints to the size of a sewage collection network.  In Brisbane the largest catchment is approximately 500 km2 with about 5000 km of sewers and an average collection time of 11 hours.  Transformations will include hydrolysis which converts CQO to CBO and also the increases % soluble CBO and bio-flocculation which can reduce % CBO.  In both instances CQO is unchanged. 

I don’t believe the redox potential would be low enough for any anaerobic CQO decomposition to CH4 and CO2, but if the sewage was maintained in an aerobic condition, organic matter could in theory be oxidised – similar to the natural self-purification in a polluted stream.  However, in practice the number of bacteria (100 times lower than in activated sludge) and contact time (hours not days) appear to be too small to have a significant measurable effect. 

A higher fraction of soluble CBO will reduce the efficiency of primary sedimentation, potentially increase the risk of activated sludge bulking and possibly result in an effluent with a lower CQO concentration (lower CQO/CBO ratio).

Thanks Michael for another excellent contribution. In fact, the maximum size that a sanitary sewer system can have in acceptable conditions has become an important issue for me since a long time ago (1990-1996), when I began working on the simultaneous optimal layout and design for sanitary sewer systems (with the selection of the optimal location of the wastewater treatment plants, the pumping systems with its short or long pressurised pipes, and the layout, design and profile of the collecting or transportation sewer pipes). That is, the three-dimensional optimization of the systems. Important economy scales and a better control on environmental issues can be reached with the suitable selection of a set of large dimension sanitary sewer systems (and corresponding wastewater treatment plants) in an integrated wide region considered as a whole.

The major problem for large or regional systems looks the sulphide control as you correctly pointed out (and as is expressed the ASCE Manuals, for example), but the information in the technical literature about the interference in the treatment looks generally very short. If you can solve the sulphide control problem, for example with air injection in long pressurised pipes in steady flow, use air-lift pumps instead of conventional centrifugal pumps (for low elevations), and use suitable design criteria, perhaps it is possible to enlarge considerably the systems and reduce significantly the number of wastewater treatment plants required. It should be observed that the costs of construction (investment), of operation and of energy of the treatment plants are frequently one of the most important parts of the overall cost involved.

COD to BOD ratio criteria are shown on HRSD’s list entitled “Wastewater Discharge Authorization Criteria COD:BOD Ratio” as amended periodically and available from the offices of HRSD or at www.hrsd.com.

https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=35&cad=rja&uact=8&ved=0CDoQFjAEOB4&url=http%3A%2F%2Fwww.hrsd.com%2Fpdf%2Fiwd_regs%2FIndustrial%2520Wastewater%2520Discharge%2520Regulations%2520Eff%2520070112.pdf&ei=2QX_VO6WA8W6uATk94HgCQ&usg=AFQjCNEpI_Zi0L08HBgM4C-PJo3CORw79w&sig2=7Hhd2UY7IhofFimdljTETQ

The characteristics of the wastewater to be treated in the wastewater treatment plant (WWTP) depend in fact from the flows accepted in the networks (that include contributions industrial, commercial, domestic and Infiltration/Inflow). Pretreatment may be required according to the regulations whenever the characteristics of particular flow entrances may be prejudicial to the biological treatment of the municipal wastewater downstream (in the WWTP). The level, characteristics, and extension of these pretreatments, in particular, can influence in fact the physical and biochemical transformations that occur downstream.

https://www.sciencedirect.com/topics/earth-and-planetary-sciences/waste-water-treatment-plant

The biochemical and chemical transformations that occur in sanitary sewer networks can have a significant impact on wastewater treatment. These transformations can affect the composition of wastewater, which can in turn affect the efficiency of treatment processes. For example, the presence of certain chemicals or compounds can interfere with biological treatment processes. Understanding these transformations is important for designing effective wastewater treatment systems.

The biochemical and chemical transformations occurring in sanitary sewer networks are highly relevant to wastewater treatment. They contribute to the removal of pollutants, reduction of toxicity, nutrient removal, and prevention of waterborne diseases. These transformations are integral to the overall goal of wastewater treatment, which is to produce environmentally safe and sustainable effluent.

The biochemical and chemical transformations that occur in sanitary sewer networks are important to consider in wastewater treatment because they can impact the quality and quantity of wastewater that needs to be treated. For example, organic matter in wastewater can undergo biodegradation, which can result in the production of gases such as methane and carbon dioxide. These gases can cause odor problems and contribute to greenhouse gas emissions.

Chemical transformations can also occur in sanitary sewer networks, such as the formation of disinfection byproducts (DBPs) during chlorination. DBPs can be harmful to human health and must be removed during wastewater treatment.

Understanding these transformations and their effects on wastewater quality can help wastewater treatment plants design and operate more effective treatment processes. For example, knowledge of the types and amounts of organic matter in wastewater can inform decisions about which treatment technologies to use, such as anaerobic digestion or aerobic treatment. Similarly, understanding the formation of DBPs can inform decisions about disinfection methods and the use of alternative disinfectants.