Municipal sludge, cake, dewatered bio-solids, and sewage: "A rose by any other name..."--it's a sure thing that such technical euphemisms for this nettlesome by-product of civilization were farthest from The Bard's mind when he penned his most famous floral reference.

But today, the treatment, transportation, and disposal of municipal waste never is far from the minds of those entrusted with keeping our environs clean and safe. Regarding same, the East Bay Municipal Utility District (EBMUD) sewage treatment plant for Oakland, Calif., was facing a possible "throughput problem." That is, after the sewage was treated in the district's multi-step digester system and dewatering centrifuge, the resulting cake or sludge had to be quickly loaded into container trucks for transportation to the district's customers. Otherwise, "inventory" might accumulate. (Yes, there are customers for everything, even sludge. More than 120,000 lb per day of Oakland's municipal by-product is purchased by fertilizer manufacturers.)

A messy problem

To keep up with supply, each truck transporting sludge from the treatment plant to the fertilizer plant had to be loaded with as much as 50,000 lb of sludge in about three minutes. This had to be done within a tolerance of ±1% because the city had to charge its customers correctly and, as with any hazardous material, keep certified records of how much went where. Furthermore, the system also had to have a price tag in line with today's lean municipal budgets, thus producing the classic engineering conundrum for the solution providers: "You want a system that's fast, accurate, and cheap? Pick any two."

The most difficult obstacle to overcome in the process was filling the trucks quickly with ±1% accuracy regardless of large fluctuations in the sludge's flow rates. These "unmodelable" fluctuations were caused by viscosity changes due to batch variants in sludge dehydration and temperature, and varying material heights (i.e. head pressure) in the loading hoppers. Unlike the constant relationship between head pressure and flow rates when dealing with liquids, municipal sludge has varying densities and flow rates ranging from that of dirt to thick soup.

The next most serious problem to overcome was measuring real-time load-out status because the programmable logic controller (PLC) would have no direct feedback indicating same. This was because:

• Trucks would not be sitting on scales during the loading process;

• Flowmeters wouldn't work in large hoppers with variable load-out orifices, and

• As the hoppers dispensed their contents, they would simultaneously be refilled, thus rendering simple hopper net weight loss calculations--as an index of load-out progress--meaningless.

Mechanical solution

Material Systems Engineers (MSE) (San Rafael, Calif.), devised a hopper slide gate system to handle the mechanics of this nasty dynamic control problem with speed, accuracy, and budget in mind. MSE's system also had to have an expected life span of at least 15 years.

To leverage the control system's capabilities, MSE uses a Diamond Seal slide gate with a diamond-shaped orifice allowing full-bore flow rates to be quickly followed by tightly metered low-flow rates. This is because the design's diamond-shaped opening decreases and increases in size much more quickly than rectangular configurations with similar actuator stroke lengths.

The control solution
MSE partnered with systems integrator, DST Controls (Benicia, Calif.), to develop the model-free adaptive control package required to accurately track, and control the dispatch of, this inconsistent blend of liquid and solid hazardous material.

Basic control requirements included establishing communications between the district's Westinghouse distributed control system (DCS) and DST's PLC system, and achieving the load-out setpoints by integrating measured net hopper weight changes with inferred hopper refill rates.

The control system also had to:

• Allow operator-entered truck identification information and loading instructions;

• Automatically enter truck tare weight;

• Time/date stamp each load-out cycle.

• Collect hopper load cell data and calculate load-out progress;

• Modulate diamond slide gates to hit load-out setpoints;

• Display load-out progress and customer identification information;

• Detect and report hopper overload alarm conditions;

• Monitor and regulate centrifuge out-flow valves and pumps; and

• Send transaction records to the Westinghouse DCS for accounting.

Major control system components include a Modicon (North Andover, Mass.) Quantum PLC, Panel Mateä Operator Interface display screen with touch-pad, Kistler-Morse (K-M, Bothell, Wa.) weight measurement system, and Miltronics (Arlington, Tex.) radar level sensors. All electronics are integrated into a single NEMA 4 cabinet. Reliability concerns and installed base in the municipal environments were instrumental in MSE's selection of Modicon as the PLC platform.

Process avoids 'free samples'
Treated sewage is partially de-watered by the plant's centrifuge system. A system of 12 valves controlled by the PLC then conveys the sludge to hoppers suspended above the truck loading area.

In automatic mode the load-out operator enters, via the touchpad, truck and customer identification information and intended load weights. The touchpad inputs this information to the PLC where it is integrated to effect slide-gate operation. On initiation of the load-out cycle from the PLC, the slide gate moves to its full open, or "course," position and initiates flow. When 70% of the desired load is reached (approx. 35,000 lb in about 2 min), the PLC moves the gate to an "intermediate" position until 95% of load-out is reached. The gate then closes to the "fine" position until 100% of the load-out weight (approx. 50,000 lb) has been completed. The PLC then closes the gate, terminating load-out all in about three minutes total.

The PLC determines gate positioning as a function of the current weight of sludge loaded to the truck. This real-time loaded-out weight is calculated from hopper weight measurements and sludge flow into hoppers. These two measurements are scanned by the PLC every second to maintain an accurate accounting of sludge loaded into the truck.

When the operator initiates the load-out sequence, the PLC takes a snapshot of the hopper weight at the beginning of the sequence (i.e., hopper tare weight). The current total hopper weight is continuously monitored as sludge is loaded into the trucks. The net weight loaded into a truck at any given time is the initial hopper (tare) weight minus the current hopper weight, plus the totalized sludge flow into the hopper from the centrifuges since that load-out began. Magnetic flowmeters at the centrifuge provide sludge-flow amounts to the PLC.

The loaded-out net weight is then used by the Modicon PLC for execution of its three primary control functions:

• Automatic and evenly distributed sludge load-out from the three loading hoppers;

• Automatic refilling of the three hoppers; and

• Transmission of transaction data, for accounting purposes, to the Westinghouse DCS.

Slide gates are actuated by hydraulic cylinders controlled by the PLC and monitored by position sensors which serve as inputs to the PLC.

To determine tare and current hopper weights, each hopper is mounted on four K-M load cells, one under each corner. The gross weight measurement is accurate to 0.1% of span (0-75,000 lb). The weight measurement system subtracts the hopper tare weight from the actual hopper weight to determine the net sludge weight in the hopper at any time.

As suggested, the system not only has to load each truck quickly, accurately and economically but also evenly. If a single 50,000 lb mound of "product" were piled in one part of the truck, vehicle handling would be adversely effected and "free samples" could be blown off the top of the cone as the truck speeds to its destination, upsetting neighbors and regulators alike. Hence, each loading platform consists of three overhead loading hoppers to evenly deposit the loads in the trucks.

Optimizing the process
Besides loading trucks properly, the PLC must also keep each hopper sufficiently refilled to maintain loading efficency. It does this by using hopper net-weight status to actuate the 12 valve system that modulates sludge transfer from the centrifugal dehydrators to the loading hoppers. The hoppers are alternately filled to a predetermined differential weight as compared to each other.

The hopper with the lowest weight is refilled first. When that hopper's net sludge weight exceeds the differential, as compared to the previous highest hopper weight, the filling sequence is redirected to the new lowest weight hopper and the cycle continues until all three hoppers are full.

The load-out operator can control the loading of sludge from the hoppers into trucks automatically or manually from the touchpad station. In either case, the screen displays net hopper weights, load out status, and customer identification information.

To perform a manual load-out, the operator turns a position switch wired to the PLC to effect the degree of gate-open desired. Needless to say, care must be taken when exercising control in this mode.

Upon load-out completion, a State of California Department of Transportation-required bill of lading with truck identification and load-out weight is printed by the system. The bill of lading is then handed to the driver and checked for accuracy at an independent truck scale by district personnel before leaving the pant.

During original system calibration, the loadout weight was periodically compared to a set of certified truck scales. To date, the agreement between the truck scales and the weight reported by the PLC has been consistently within 0.5%. This means trucks keep rolling out from under loading gates fast enough to spare Oakland that mother of all municipal waste problems--"accumulated inventory."