INSTRUMENTATION SYSTEM DESIGN FOR A continuous anaerobic bioreactor
Katherin Indriawati
Instrumentation Engineering Laboratory
Teknik Fisika ITS
Introduction
The anaerobic digester process presents very interesting advantages compared to the classical aerobic treatment. It is because this process has some advantages: it has a high capacity to degrade concentrated and difficult substrates, produces very few sludges, requires few energy and in some cases it can even recover energy using methane combustion. But in spite of these advantages, the anaerobic digester are still very rare at the industrial scale, probably because they are known to become easily unstable under some circumstances like variations of the process operating conditions. This problem is more likely to occur in a continuous bioreactor. Nevertheless this drawback can be overcome by associating a monitoring and controlling procedures that allows to enhance the stable performance of the on-line wastewater treatment operation. Hence, the instrumentation system must be developed in order to conduct the monitoring and controlling actions. This poster describes the instrumentation system that have been developed in the anaerobic digester miniplant at Instrumentation Engineering Laboratory, Teknik Fisika FTI – ITS.
Method
The miniplant is an anaerobic upflow digester. The reactor is a circular column of 60 cm height, 17 cm diameter. The effective volume of the medium is 10 liters. The experiments were performed with tofu waste water obtained from the local tofu manufacturer in the area of Surabaya, Indonesia. This substrate, neither sterile nor homogeneous, was filled to the feed tank (3 liters) which are connected to the bottom side of the reactor by a plastic transparent piping of 0.25 in. The effluent flew down from the top side of the reactor to the container which was located in lower position than the reactor. While the biogas product naturally flew out from the reactor to the gas collector through a transparent piping of 0,25 in.
The method for collecting and measuring a biogas product of the digester is based on one called collecting over water or displacement of water. The setup for the collection of a biogas over water involves a gas collection container filled with water and inverted in the reservoir of water. The gas collector is a glass tube of 250 ml. The gas evolved from the anaerobic process is collected by attaching one end of a hose (a plastic transparent piping) to the reactor and inserting the other up into the inverted gas collection glass tube. As the gas is created, it will displace water from the glass tube. The volume of gas can be determined by the amount of water that was displaced by the gas. For these, the reservoir of water is equipped with level sensors that allow to obtain the rate of level change. The volumetric flow rate of the biogas product is proportional with these rate of level change. Figure 1 describes the technique of biogas flow rate measurements.
Figure 1. The biogas measurement principle
pH measurement is only performed in the effluent stream, that is by dipping the pH sensor in the tank of effluent. It is intended to provide information about the condition of pH in the bioreactor. pH sensor used is a glass electrode. Because the output of this sensor is very small, the signal conditioning circuit is required to make the data signal can be used at a later stage.
The measurement of flow rates of biogas and effluent pH then used to create a monitoring system and control system for the anaerobic digester plant. The monitoring system is built to detect the operating conditions that occur in the bioreactor. While the control system is built to enhance biogas production and prevent washout.
The operating conditions detected by the monitoring system is normal, overload, and underload. Because it uses two measurement variables, ie biogas flow rate and pH of the effluent, then multivariate method is used. One of multivariate methods used in this study is the principal component analysis algorithm. These algorithms reduce the multivariate data into a new variable called a latent variable. Then using hotelling T2 values of that new variable, the operating conditions of bioreactors can be determined. Determination of the operating conditions is done by using fuzzy logic as a decision maker.
In this study, the control system was built to control the pH and flow rate of biogas. pH control is intended to prevent washout condition in the bioreactor. Control of pH is done by manipulating the flow rate of NaOH solution that is fed into the bioreactor. Biogas flow rate control is intended to maintain the biogas production at the desired value. This is done by manipulating the flow rate of liquid waste out into the bioreactor. For the pH control system, on-off algorithm is used, whereas the biogas flow rate control system used proportional algorithm.
Results
Installation of instrumentation at the anaerobic digester system as a whole is shown in figure 2. Some of the results of the monitoring system in detecting the operating conditions of the bioreactor is shown in Figure 3. While the results of pH control and biogas flow rate is shown in figure 4 and 5.
Figure 2. The whole instrumetation system for bioreactor
Figure 3. The monitoring results of bioreactor condition
Figure 4. The level controlling result of the gas collector reservoir as a representation of biogas flowrate controlling
Gambar 4. The pH controlling result of anaerobic bioreactor