Advantages of plug flow in activated sludge process (ASP)
The primary design and operational target of the ASP for BOD removal is obtaining good solids settling properties in secondary clarifier. For instance, food to microorganisms (F/M) ratio are limited at 0.2-0.4 g BOD/g MLSS/d in a typical ASP to obtain biosolids with a good sludge settling properties although microorganisms can accommodate much higher F/M ratio.
One of the drawbacks of ASP is a diminished solids settling in secondary clarifier when excessive filamentous microorganisms grow. The exact causes of the filamentous bloom are not clear, but one of the popular theories suggests that the filamentous bloom can be linked to the excessively low F/M ratio that occur at least temporary basis in municipal wastewater treatment. Due to the large surface to volume ratio of rod shaped filamentous microorganisms, filamentous microorganisms may have better chance to absorb food than floc-forming microorganisms.
Meanwhile, solids settling issues are commonly experienced with CSTR type aeration tanks especially in municipal wastewater treatment, where wastewater COD can go down excessively. The mechanisms are not clear, but the filamentous bloom in CSTR is often attributed to the fact that wastewater instantaneously dilutes in the reactor and all the microorganisms are exposed to food scarce condition.
In order to inhibit filamentous bacteria growth, plug flow based aeration tanks were devised. In plug flow reactor, aeration tanks are designed as long narrow channels through which the mixtures of fresh wastewater and return activated sludge (RAS) flow. Since the fast growing floc-forming microorganisms absorb much more foods than filamentous microorganisms in upstream, little foods are left over for both types of microorganisms in downstream. Alternatively small aeration tanks are serially connected to maximize plug flow effect by minimizing back- and forward-mixings in the channel. While there is no perfect CSTR or plug flow, all the process designs lie somewhere in between the two extreme reactor types.
Some drawbacks of plug flow type aeration tanks include: 1) the high oxygen demand in upstream makes it hard to dissolve enough oxygen due to bubble coalescence in the area to support biological COD degradation, 2) due to the bubble coalescence and high surfactant-like molecules in wastewater, oxygen transfer efficiency is low in upstream, 3) the high F/M ratio causes higher diffuser foulings in upstream, which again makes it harder to dissolve enough oxygen in the local space, 4) etc.
Plug flow in MBR
The primary reasons why many ASP adapt plug flow are to improve sludge settling in secondary clarifier, but this is an obsolete concept in MBR since MBR does not have secondary clarifiers. MBR sludge typically contains high level filamentous organisms probably due to the low F/M ratio as discussed here, but it is not a significant issue as long as the filamentous microorganisms do not cause significant foaming issues. In addition, filamentous microorganisms are arguably not a primary concern for membrane fouling since biopolymers including polysaccharides and proteins are the more significant factors in this regard.
One potential advantage of plug flow in MBR is that effluent TKN can be better controlled by preventing them from short circuiting especially in combined tanks where aeration tanks and membrane tanks are merged. However, if separate membrane tanks exist in addition to aeration tanks, no direct TKN leaks occur. Since COD is quickly absorbed by heterotrophs in biological wastewater as can be seen in contact stabilization processes, COD short circuiting is not noticeable even in combined tanks.
Strict plug flow designs often causes problems in MBR due to the excessively high local F/M ratio in upstream. Fig. 1 shows one example, where small aeration tanks are serially connected. Perhaps design engineer’s intentions are maximizing the reaction rate in the first tank while polishing the treated wastewater in the subsequent tanks. It might be also expected that the growth of filamentous microorganisms can be minimized and as a consequence membrane fouling and/or foaming can be reduced.
Fig. 1. Plug flow MBR design using serial multiple CSTR.
However, the serial reactor design can hamper the system operation significantly especially when F/M is excessively high in the first reactor. The high F/M ratio causes high oxygen uptake rate (OUR), which in turn makes it hard to maintain a proper DO. The low DO in aeration tank can simulate microorganisms produce more polysaccharides and proteins that can accelerate membrane fouling as shown in Fig. 2 (Kang, 2003). It was found that TMP started to increase abruptly as soon as DO started to decline by pausing biological aeration while membrane scouring was performed using nitrogen gas.
Fig. 2. Variation of: (a) the TMP profile and (b) the DO concentration at each SBR phase during the submerged microfiltration (Kang, 2003).
For example, while the average F/M ratio stay at a proper level such as 0.1 g BOD/g MVLSS/d, the local F/M in the first tank can be >0.5 g BOD/g MLSS/d in serially connected aeration tanks (Fig. 1). The high F/M ratio not only increases oxygen uptake rate (OUR), but also reduces oxygen transfer efficiency (OTE) as discussed here. As a result, DO in the first tank can be excessively low. As a consequence, microorganisms produce more biopolyers (Kang, 2003), which can cause excessive foaming as shown in Fig. 3. The intense aeration in the first aeration tank also contribute to the foaming. Most importantly, the biopolymers produced in the first tank may not be fully degraded before they reach the membrane tank and enhance membrane fouling due to their lowly degrading nature.
Fig. 3. Excessive foaming in the first aeration tank in serially connected CSTR. The first aeration tank takes 1/7 of the total aerobic volume including membrane tanks in this case.
Fig. 4. Continuously stirred tank reactor (CSTR).
Based on mass balance around a tank, equation (1) can be written, where left term represents substrate accumulation in the tank and right term represents incoming and outgoing substrate. This equation can be solved for the boundary conditions of (t1, S1) and (t2, S2). Equation (4) is the final outcome.
If feed water with a substrate concentration of S0 starts to be fed at a flow rate of Q to a CSTR with a volume of V, the normalized substrate concentration against feed concentration (S/S0) increases as Fig. 5. When the normalized time against HRT (t/θ) reaches 1, S/S0 reaches 63% of the maximum level. At t/θ = 3, S/S0 reaches 0.99. In other words, the substrate concentration in the tank reaches 95% of the feed concentration after the tank is replenished three times with fresh feed water. Conversely, once feed water is replaced with clean water at t/θ=10, S/S0 decreases to 37% of the maximum at t/θ=10 and it decreases to 5% of the maximum at t/θ=13
Fig. 5. Normalized concentration in CSTR as a function of normalized time where feed water with a concentration, S0, comes in at time zero and the feed concentration drops to zero at t/θ = 10.
Plug flow is an opposite concept of CSTR. In ideal plug flow reactors, feed is supplied to a thin and long channels such as pipes, where anything entered the reactor first comes out first. In real world, however, some extent of forward and backward mixings are inevitable due to the molecular diffusion and the lagging liquid flow near the reactor wall.
As can be seen in Fig. 6, when a step change occurs in feed concentration under a constant flow rate, effluent concentration lags behind with a delay time equivalent to hydraulic retention time (HRT) of the reactor. Some substrates comes out of the reactor
Fig. 6. Relation between feed concentration and effluent concentration in plug flow reactor.
© Seong Hoon Yoon