Why do you think this waste water treatment plant failed? Examples of failed waste water treatment plants are abundant throughout low income countries. One common reason why is people try to use them for the treatment of fecal sludge even thought they were designed to only treat waste water. In this module, we will learn about ways that fecal sludge and waste water could be co-treated together. Following this module, you will be able to understand key considerations and potential impacts of co-treatment in sewer-based waste water treatment systems explain the importance of influent characteristic on modeling of waste water treatment plant performance and understand that waste water treatment plants are complex systems with processes that can be upset. As you are well aware by now, fecal sludge is different from waste water and cannot just be treated the same. It is much more concentrated, variable, and is partially degraded. There are possibilities for co-treating fecal sludge and waste water, but the possibility of failure is high and the consequences to failure are very significant as then neither waste would be receiving treatment. Waste water treatment plants are not black boxes they are basically very complex factories that are receiving polluted water and turning it into clean water and sludge. The basis for stabilization treatment are very diverse from Lagoons, to activated sludge, to trickling filters as for de-watering from drying beds to, centrifuges, to screw presses. What is possible for co-treatment if anything will depend on the type and design of technologies, the quantities and characteristics of waste water and fecal sludge including most importantly is the treatment plant currently at capacity or not and existing infrastructure. It will also require a competent waste water engineer who also understands the complexities of fecal sludge to model any proposed modifications, and also verify with in field testing. If you have a waste water treatment plant that is currently under capacity there are three ways you can investigate co-treatment with fecal sludge. One, de-water the fecal sludge and send the resulting solid and liquid streams for treatment with the solid and liquid waste water streams. Two, treat the fecal sludge together with the bio-solids starting from the de-watering stage. And three, directly treat the fecal sludge with the influent waste water. In general, three is not recommended and it needs to be stressed that fecal sludge should never be added directly into secondary waste water treatment. The preferred option for co-treatment is starting with de-watering of fecal sludge. This could include settling thickening tanks drying beds, or mechanical de-watering. If you're looking for fecal sludge de-watering solutions prior to the development of additional infrastructure temporary options like geo-textile bags could be considered. The effluent could then either go into primary or secondary treatment or co-treatment with the waste water depending on the characteristics and the solids could then be co-treated together with the waste sludge or bio solids. With this option, ways to increase de-watering and drying of both solid streams could be investigated thereby increasing the treatment capacity of the plant for example, with conditioning the bio-burn pelletizer mixing of sludge on drying beds, or thermal options. The second option is very similar, with the fecal sludge going directly to the waste water solids de-watering step of the treatment plant, for example, de-watering on drying beds. This figure is from the 1984 US EPA publication on how to co-treat septic sludge in waste water treatment plants. The allowed loading depends whether the treatment plant is at capacity and what type of treatment technologies are employed. Now think about this, if only 10% of operational capacity within the aerated lagoon you can still only add 3% of the total volume as septic tank sludge. For treatment plants that are operating at 90% of their capacity, the allowed loading volume metrically is less than one half of 1% of the total influent volume. But what is even more important to consider is that these calculations were based on selected values for septic tank sludge in the united states in the 1980's. If we put the design value for that figure onto our figure It's obvious that the diverse range of fecal sludge that we're working with in general a lot more concentrated. This means the possible loadings would have to be adjusted to be even much lower. This stresses the importance of when implementing anything that tests need to be made on local sludge that will be treated to understand settling, de-watering behavior and the characteristics and quantities that will be treated. Chapter 9 in the fecal sludge management book covers detailed modeling in more depth, it covers the impacts that fecal sludge with different strengths, concentrations degradable organic matter and also inert solids have on the loading of the waste water treatment plants. Impacts on operations that can lead to failures include reduced effluent quality, maxed out aeration resulting in an aerobic process turning anaerobic incomplete oxidation, filamentous bacteria and overloading of settling tanks and clarifiers. Overloading of anaerobic systems can also lead to inhibition of biological processes and increased sludge production maxing out solids handling. In addition the US EPA recommendations are for waste water treatment plants serving areas that are mostly sewered. In areas where there is any significant coverage by on-site systems, which is the case in most cities we are working in in Asia, Latin America and Africa, there needs to be separate designated treatment of waste water and fecal sludge. For management purposes it can effectively take place at the same location but it needs to be carefully planned out. As covered in other modules, there are plenty of options for the treatment of fecal sludge and even many more possibilities like this process flow for treatment of septage, or septic tank fecal sludge with an ABR filter and wetland from the United States. Finally, one of the most important considerations when co-treating is when and how to add the fecal sludge. Fecal sludge should never be discharged to the sewer unless it has been de-watered first. And the effluent is going to the sewer. Sewers are designed for the gravity flow of waste water and thicker fecal sludge does not have the same flow properties resulting in blockages and overflows. To avoid shock loadings the prefered method would be a continuous flow at a rate that is proportional to the waste water influent. To illustrate these points, I will show you two different examples. The first in the city of Manila, the capital of the Philippines is a treatment plant operated by Manila water which is a successful case study of example one, de-watering of fecal sludge and then co-treating the liquid and solids with the separate solid and liquid streams from waste water. This plant receives 2 million liters per day of conventional waste water and 0.8 million liters per day of septic tank sludge or septage. This is the process flow for the treatment plant. The waste water treatment chain is conventional activated sludge. The waste water is treated by primary sedimentation activated sludge, secondary sedimentation and then disinfected prior to discharge. For the fecal sludge management, vacuum trucks empty septic tanks deliver the sludge to the treatment plant, there is separate pre-treatment for the fecal sludge and then it is de-watered together with the solids from the waste water stream with the addition of polymers and a screw press. The co-processed waste water and fecal sludge solids are then transported off site by an approved contractor for treatment with drying beds and co-composting with rice hull and then used as a soil amendment. The second example is a project developed by the federal university of Minas Gerais, Brazil, as a part of an international program financed by the Bill and Melinda Gates foundation for the project coordinated by the UNESCO-IHE. This is also an example of number one de-watering fecal sludge and then co-treating the liquid and solids together With the separate solid and liquid streams from the waste water. This project took place in Belo Horizonte in the state of Minas Gerais, Brazil, which has a well developed sewer network together with poor areas served by septic tanks. Domestic waste water treatment occurs in three parallel French system treatment wet-lands that have been operational since 2009. They operate in a weekly cycle of feeding and resting. Waste water is directly loaded in batches onto one of the beds following screening and grit removal and they achieve a mean treatment efficiency of 80% of COD and 84% of TSS. in 2013. the researchers evaluated using the wetlands system for co-treatment with fecal sludge from septic tanks, they did this by loading unit one with fecal sludge and then alternating different loadings on units two and three with the fecal sludge leachate from unit one together with the raw waste water. The research was focused on whether the wetlands system could effectively co-treat waste water and fecal sludge and whether ponding or retaining the leachate within the fecal sludge bed for one week could increase treatment performance. Even with the reduced area for waste water loading with only two units being used instead of three, almost the same treatment efficiency was observed for waste water of 72% for COD and 84% for TSS. Ponding in the fecal sludge unit also had the greatest impact on improving treatment performance with removals for the fecal sludge of COD of 90% and TSS as 71%. Based on their experience this type of system is effective for co-treatment of fecal sludge and waste water with one unit designated for fecal sludge with the leachate going to co-treatment with waste water. This full scale pilot could provide fecal sludge treatment for around 2500 population equivalence with the 30 square meter surface area for receiving the sludge. And with available land this could readily be scaled up as necessary. In this module we've learned about the complexities and dangers of co-treating fecal sludge in waste water treatment plants and the importance of understanding influent characteristics on the complex functioning of treatment plants. Do you know of an example in your city? One that is a failure and why, or another that's an example of success? You should add it to the forum so we can include it for discussion. Thanks for joining and see you next time.
