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In anaerobic waste fermentation processes, the gas composition and production rate are key indicators of reactor performance. Several systems have been devised to measure the gas production rate (flow rate) in the range usually found in laboratory-scale anaerobic digesters (1 -100 mL/min), most of them using liquid displacement measurements. Recent typical gas measurement devices have been described by Van den Berg,' Moletta and Albagnac,2 Glauser and coworkers,3 and Erdman and D e l~i c h e .~ However, liquid displacement techniques are often inconvenient for various reasons, namely gas solubility in the displaced liquid, liquid evaporation, difficulty of operation and automation in industrial environments. The development of a simple fast response gas metering system which avoids these problems thus seemed a highly desirable goal. The work described here was therefore undertaken: 1) to design a low-cost, electronic gas flow measurement system which could simultaneously monitor the gas production PRESSURE TRANSDUCER n rates of several digesters and 2) to develop a microcomputer interface for data acquisition and processing. THE FLOW SYSTEMThe flow metering system, as illustrated schematically in Figure 1, consists of a three-way solenoid valve connected to a pressure transducer, a glass bulb (ballast), and the vapor space of the monitored reactor. The three-way (stainless steel) solenoid valve is set so that the two normally-open ports (1 and 2) communicate with the transducer and ballast. The third port is normally closed; in the open position, it vents the ballast and transducer to the atmosphere.The operation of the flow system is quite straightforward. With the third port normally closed, the pressure in the reactor and ballast increases progressively. As the transducer VENT VALVE Figure 1. Schematic description of the flow systems used to measure gas flow rates. BIOPROCESS

Biological removal of iron to produce drinking water was established in a pilot plant treating raw water with a pH of 5.7. The objective was to evaluate the use of oxidation-reduction potential (ORP) as a control tool and determine its relationship to dissolved oxygen (DO) and residual iron concentration in filtered water from an operating biological filter. Results showed that above a low minimum value of DO, residual iron concentration and ORP were not affected by varying the DO level. A non-linear regression was established to correlate total residual iron concentration to ORP with an R2 of 0.8848. This correlation can be used to predict iron concentration when ORP is in the range 300 to 470 mV. Below this range, total residual iron is greater than or equal to 3 mg/l and above, total residual iron is less than the French regulation limit of 0.2 mg/l. Pilot plant operating conditions were implemented in the primary filter of an industrial plant in France, improving iron elimination and doubling the length of the filtration cycle.