There are numerous well-proven options available for level determination instrumentation. Instrumenting treatment processes present some unique challenges. Some of these include the ability for wastewater or residuals to clog and blind instruments. Level determination in an anaerobic digester presents these problems as well as others including foaming, surface agitation, and operation in a methane environment. The radar level instrument provides a new technology that has the potential of overcoming these problems to provide reliable and accurate level readings in Digesters.
The Los Angeles County Sanitation Districts operate 11 wastewater treatment plants. The largest plant is the Joint Water Pollution Control Plant (JWPCP) located in Carson, California. This plant has an average flow of 320 MGD with a design capacity of 400 MGD. The major unit processes are Primary Sedimentation, Pure Oxygen Activated Sludge, Anaerobic Digestion, and Biosolids Dewatering.
When this work was initiated in 2001 JWPCP was operating 17 circular anaerobic digesters, which are 125 feet in diameter with a capacity of 3.75 million gallons. There were also 23 rectangular digesters which had a capacity of 0.75 million gallons each. These rectangular digesters were at the end of their useful service life and a project to construct 7 additional circular digesters of similar design to the existing digesters was due to begin shortly. The circular digesters have a flat fixed roof and are mixed with five draft tubes. These tubes mix the primary and thickened waste activated sludge using recirculated digester gas. These digesters are operated in the mesophilic temperature range (93ºF). The physical digester characteristics and sludge characteristics lead to the formation of a layer of foam on top of the digester. The depth of this layer of foam can vary from several inches to several feet. The depth of the foam layer can vary in the same digester from day to day. On rare occasions there can be hydraulic or other problems such as partially blocked runoff lines or leaking feed valves that would change the balance of foam in the digester over short periods of time. The normal static sludge level in JWPCP’s digesters is about 4 feet from the roof. This typically provides ample headspace to prevent foam or sludge from being pushed out of the digester seals. Never the less the previously mentioned hydraulic problems have on rare occasion caused a digester to overflow. It is always the intent of the Districts and JWPCP Operators to prevent such overflows from occurring.
Not only are overflows low frequency events but the time between the event trigger and occurrence can be quite small. Plant operators for routinely check the digesters for proper functioning and level. JWPCP is manned 24 hours a day with 3-operator work shifts per day. Operators use a glass porthole type device to visually examine the level of sludge and approximate foam depth in the digester, see Figure 1. The treatment plant operators have developed considerable expertise in estimating the levels visually. However, since there is a considerable time interval between operation checks they are not always sufficient to detect precursors to an event. Instrumentation that detected the level of foam or liquid in the digester reliably would be of great benefit. An instrument, which would only measure the level of liquid in the digester, would miss events in which the foam layer had increased in depth so that it was close to the digester roof but the liquid level was normal. Typical instruments such as ultrasonic level, pressure measurement, floats, level switches, etc. either respond only to liquid level or would potentially be compromised by solids build up on the instrument. Ultrasonic type level instruments were installed on several of these digesters many years ago but had problems with absorption of the signal by the methane in the headspace and could only detect liquid levels. For these reasons the instruments fell into disfavor with the operators and were abandoned.
Radar level instruments using microwave frequencies have only been on the market for about 10 years. These instruments claim accuracies of 0.1 inch and are thought to be highly resistant to failure. In addition, they have the ability to operate in a carbon dioxide and methane atmosphere and are capable of detecting foam if the density is greater than 0.1 g/cm3. Although no data on digester foam density was available it was thought that the solids content of the foam would render it visible by the instruments.