CTB3365x Introduction to Water Treatment

Transcription

1 CTB3365x Introduction to Water Treatment W3a Oxidation fundamentals and kinetics Merle de Kreuk Do you already question yourself how soluble components are removed from sewage? And what the brown suspension is that you see in the process units? Today we will start discussing the heart of many sewage treatment systems: the biological treatment. My name is Merle de Kreuk, Assistant Professor Sewage Treatment and Digestion and I will introduce the biological processes to you. 1

2 In this lecture I will tell you about the beautiful world of microbiology. What I would like you to remember is the functionality of bacteria in sewage treatment, predict which group of organisms will become dominant when their growth curve is known and calculate substrate conversion rates. In the biological part of the sewage treatment plant, biodegradable constituents are converted into harmless components, such as dinitrogen gas, Carbon dioxide and water. Important processes are the removal of BOD, nutrients and incorporation of suspended solids in the activated sludge flocs. All these processes are performed by microorganisms. The beauty of microorganisms is that everything is everywhere, but the environment selects. These famous words come from professor Baas Becking, inspired by professor Beijerinck, founder of the Delft school of microbiology. It forms the base of many current and new sewage treatment technologies. Bacteria belong to the Prokaryotes, microorganisms that do not have a nucleus, mitochondria, or any other membranebound organelles. In other words, all their intracellular components, as proteins, DNA and metabolites are located together in the same area enclosed by cell membrane. Their overall composition is more or less the same. Therefore we can use a general structural formula 2

3 to describe the bacteria in our systems. There are several temperature ranges in which bacteria can survive. Organisms that have their growth optimum between -10 and 25 C we call psychrophilic, with their optimum between 20 and 45 C we call mesophilic and the organisms that like high temperatures, between 40 and 122 C we call thermophilic. Even hyperthermophilles exist, with growth optimum around 106 C. In sewage treatmentplants most bacteria are mesophilic, although in sludge digestion also thermophilles can be used. So now you know that we treat our sewage with micro organisms. The chemical processes, that actually convert the contaminants and that occur within the microbial cells are called metabolism. There are two basic types of processes: Catabolism, or dissimilation, is the process in which chemical components are converted generating energy. This energy is used for the assimilation of new cell material and other cell functions: the anabolism. All energy converting reactions in microorganisms, apart from the phototrophic ones, are oxidation-reduction or in short redox reactions. They occur in a variety of electron donor, electron acceptor combinations. As long as the reaction is thermodynamically possible, most probably they can also be catalyzed biologically. Can you think of a well known electron acceptor.. or oxidizer? 3

4 Considering all this, we need a few conditions for growth of microorganisms: First we need an energy source, an electron donor and a carbon source. In many conversion processes, one component can be all three at the same time, as glucose in chemo-organo-heterotrophs. In redox reactions, besides an electron donor, also an electron acceptor is needed. In other words, if one component is oxidized, another one needs to be reduced. In sewage treatment we use different redox conditions, indicating the electron acceptor present: This is aerobic, when oxygen is the main electron acceptor. Anoxic, when nitrite or nitrate is present as electron acceptor, or Anaerobic conditions, when oxygen, nitrite or nitrate are absent and any other organic or inorganic component serves as electron acceptor. This could be sulfate, carbonate, metals or even several organic compounds. Which were the four conditions we need again for microbial conversion? Energy and Carbon source, Electron Donor and Electron acceptor Well, besides the four mentioned conditions, also a nitrogen, fosfor and sulphur source is needed for bacterial growth, as well as other macro, and micro nutrients and growth factors as vitamins and amino acids. The first four parameters mentioned determine the terminology for microbial metabolism as can be seen in this table. For example Auto-trophic means self feeding, in other words, organisms do not need an organic carbon compound synthesized by another organism, but they can use CO2 as carbon source. If we now translate what we learned in the past minutes to a biological system, what would we see over time, measuring substrate and methane production rate or oxygen consumption rate in an active system? If we would have a bottle with activated sludge, aerate the bottle, and dose substrate, we would see that the substrate is converted over time, while the oxygen is consumed. When the electron donor is finished, also the consumption of the electron acceptor slows down severely. Under anaerobic conditions a similar trend could be observed. When substrate is dosed, its product will be produced. The production of methane stops when substrate is finished. 4

5 When we translate all this to a sewage treatment plant it would mean that BOD is converted to products and bacterial cell mass. If we focus now on the growth of biomass in a batch fed system, we could see the following. First, organisms need to adapt to the substrate, produce enzymes, so cell growth is not occurring yet. We call that the lag-phase. Hereafter, biomass starts to grow exponentially and substrate is consumed. When the substrate is nearly finished, bacterial growth slows down, where after the biomass will decay. The amount of biomass that is produced per amount of substrate fed is called Yield. The actual substrate consumption rate, and thus the biomass production rate depends on several factors, as the concentration of biomass, electron acceptor and electron donor, as well as the temperature, presence of inhibitors and other environmental conditions. In sewage treatment conversion processes, the Monod equation is often used to describe kinetics. It says that a changing biomass concentration in time, depend on the actual growth rate of the bacteria and the current biomass concentration. The actual growth rate is a function of the maximum growth rate of the organisms and the actual substrate concentration together with an affinity factor Ks. Ks is the substrate concentration at which the actual growth rate of the organism is half of its maximum growth rate. 5

6 As said before, the substrate conversion rate is related to the actual growth rate via the biomass Yield, as expressed in this equation. The substrate concentration as described in the simple Monod function, can be substituted with all growth factors mentioned before. All rates are temperature dependent too. This difference in actual growth rate is used in mixed culture systems that consist of many different species. By changing process conditions and adjusting the sludge age of the biomass present, the fastest growing organisms can be kept in the system, while the slower growing organisms will be outcompeted. This is illustrated in this graph. Two species are competing for the same substrate in a biological system. Which organism will be dominant at low substrate concentrations? And which at high substrate concentrations? Think about it and take the chance to discuss it at the discussion platform! 6