Control by Observation

The staff at a small New York treatment plant takes an ingenious approach to give operators a visual assessment of trickling filter status and performance.
Control by Observation
The trickling filter at the Corning plant enables visual observation of biomass growth.

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Operators at the City of Corning (N.Y.) Wastewater

Treatment Plant knew the advantages of visually monitoring biomass growth in their submerged rotating biological contactors (SRBC).
By watching the biomass on plastic discs rotating in and out of the wastewater flow, they could help maintain healthy fixed biofilm, and respond to biomass changes trending toward overgrowth, undergrowth, nitrification activity, odd growth, slough-off, or signs of structural media failure.

The plant’s 90-foot-diameter trickling filter, on the other hand, was a “black box,” operated only by influent and effluent comparisons, which usually gave results too late for an operator to maintain steady-state treatment conditions. The team perceived that they could avoid ponding, septicity, odors and poor treatment if they had a visual early warning of biomass growth changes, as with the SRBCs.

Therefore, the Corning operators found a way to open their trickling filter with a hoist and harness system. This allowed them to lift one of the modular media blocks and expose the active biomass underneath. The result was SRBC-like observation, revealing “first-stage” biomass color, density and odor and a representative weight.

Operator response to these newly available parameters led to effective control programs of routine flushing and occasional flooding of the plastic media. Today, the Corning plant avoids common trickling filter problems: overgrowth, odors, flies and others, and sustains steady-state treatment conditions year-round.

The lessons learned by investigating the internal growth of trickling filter-fixed media biomass apply to all fixed-media operations. Even operators of the 40-percent-visible SRBC rotating drum, which clearly reveals the status of its surface biofilm, should consider biological activity deep in the plastic wedge toward the shaft’s center, as it may differ significantly from surface activity.

The Corning operators’ experiments with slough-off options to maintain a healthy fixed-film growth shows that steady-state trickling filter operation can be achieved, even through seasonal changes.

Upgrading treatment

In 1994, the City of Corning and the state Department of Environmental Conservation (DEC) agreed to change the treatment plant’s SPDES permit to begin reducing ammonium through nitrification.

Accordingly, city officials contracted for engineering and construction modifications to the plant. Besides extending the fixed-film process with air-driven, 60-percent-submerged SRBCs, the project replaced the existing trickling filter’s gravity distributor arm (gravity-driven by discharge water) with a motor-actuated rotating distributor (MARD). In 1999, the trickling filter limestone media was replaced with modular plastic blocks, as bio fouling and associated septic odors occasionally occurred within the rock media.

The engineering firm provided training and written instructions for the plant team on the operational flexibility of using a motor-driven distributor and the treatment levels to be expected by using high-rate plastic media. After startup, odor was eliminated and, as expected, treatment improved as well.

However, after a year of continuous success, odors again began to come from the filter, and the operators felt uncomfortable simply having “run” the filter and never having “operated” it. Because treatment remained adequate, they did not have to operate it, but now they questioned whether elimination of bio fouling and improved treatment would continue for the long term.

Neighboring plants that used trickling filters or bio towers provided tours and answered questions about their operation and maintenance experience with fixed media, and articles in professional journals on the topic further stimulated the operators’ interest in properly managing the trickling filter.

Taking action

The Corning operators were propelled to action when the regional DEC inspector told of an incident at a neighboring plant that had been upgraded to the same modular plastic blocks in place of limestone media: The plastic media plugged and collapsed under its own weight.
The team worked to identify materials and equipment needed to hoist and weigh one of the trickling filter fixed-media blocks. They selected a hoist with a manual winch, along with an independent spring scale for weighing the block.

After management supported the purchase, the operators designed and constructed the lifting harness and installed the hoist (a davit crane). They began collecting data by routinely harnessing, hoisting and weighing a new replacement block — and in this way monitoring the inside of a trickling filter fixed-film media for bioactivity.

The project is self-sustaining in that high-quality materials were used and funding fell within the annual operations budget capacity. The benefits of avoiding poor treatment or catastrophic failure in the trickling filter’s structural media will far outweigh the initial and replacement costs of the project.

Visualization is power

The Corning treatment plant uses fixed media of a modular plastic shape in its trickling filter and 10 SRBCs. The trickling filter mechanically distributes the wastewater flow evenly over its fixed media, and the SRBCs mechanically rotate their media in and out of the flow. Either way, pollutants are consumed by contact with the zoogleal film of bacteria and other organisms growing on the media.

Experience with the SRBCs showed the Corning operators that observation is a key tool in maintaining healthy and efficient fixed media. They easily see changes in biomass growth on the drums as it responds to varying ratios of nutrients in the wastewater. They can observe color (from gray to brown), density (shaggy to thin), splotches or other oddities in growth, slough-off, or clinging debris. They can also take scrapings for microscopic inspection from any stage of treatment along the train (multiple independent rotating drums) of the SRBCs.

The weight of a drum (a sensor converts downward shaft pressure to pounds) is also an indicator of biomass density and health: Weight correlates so well with biomass appearance that eventually operators can estimate drum weight just by observation. This reveals whether the bacteria are growing, sloughing, nitrifying or going septic, allowing operators to manage SRBC operation proactively and manage slough-off.

The tricking filter media, on the other hand, is housed in a tank structure and receives its feeding as water trickles down through it. Only the weather-beaten surface is visible, and it is not representative of the biological activity underneath. Therefore, operators cannot directly observe biomass health indicators.

Operators typically respond to trickling filter needs when influent analysis compared to effluent shows signs of poor treatment, settleability, or low dissolved oxygen. When major symptoms appear — such as odor, ponding of water on the surface, or surface depressions that indicate crushed lower media from overweight biomass — it is too late for operators to respond, as treatment efficiency is lost and a crisis mode kicks in.

Devising a remedy

The Corning team believed they could maintain a healthy biomass and efficient treatment by varying feed rate, arm speed, flushing frequency or flooding routine if they could observe the growth of the biomass to know when to respond.

Based on their SRBC experience, it seemed logical to open the trickling filter and hoist a modular media block from the top of the filter bed so as to observe and weigh it. The challenge was finding a way to do that safely.

In discussions with the plastic modular block manufacturer, the plant team found that 12 pounds of active biomass per cubic foot was the recommended limit if the blocks were to deliver 20 years of service. Since each block measures 4 feet long, 2 feet wide and 1 foot high (8 cubic feet), the recommended weight limit was 96 pounds.

The manufacturer’s representative knew of no block hoisting and observation devices, but the Corning team decided to go forward and see if they could open the “black box” for more efficient process control.

The Corning trickling filter consists of a 90-foot-diameter concrete tank with an 8-foot, 6-inch sidewall depth, buried with 2 feet exposed above grade. With the media block joints staggered in six 1-foot layers and the whole resting on 8-inch-high supports that create an underdrain, the 6 feet of media stacks roughly to grade level.

In October 2001, operators pushed a 1 1/4-inch PVC pipe through the center of one of the top blocks and then weaved through it, to the four corners, a 3/16-inch stainless steel cable. They brought the ends together to wrap the block’s rectangular shape, creating a four-point connection for a balanced lifting point.

A small self-standing davit crane with hand winch, mounted at ground level just outside the trickling filter tank wall, hoists the test block. The variable-frequency-driven motor that rotates the distributor arm is turned off or slowed down to allow plenty of time to remove the block from its position safely and without interfering with the arm.

When the media block is hoisted, the exposed undersides look nothing like the barren top surface of the trickling filter, which is exposed to sun, weather and hydraulic pounding from the distributor arm’s discharge.

The active biomass reveals its color, texture and density, and the porosity of the block’s passages is observable. Scrapings of the zoogleal film for micro scopic examination are available from the lifted block, the sides of neighboring blocks, and the tops of the layer of blocks below. Observations made in this way result in standard operational responses:

Heavy white gel-like streaks with a black underlayer located mostly in the media crevices indicate Beggiotoa on a septic anaerobic layer, and the response is to increase flushing.

Clumps of lint-like masses on otherwise nearly clean media walls indicate filter fly larvae grazing on biofilm, and the response is to flood the filter.

Smooth, consistent biofilm indicates a healthy condition, and the response is to continue monitoring.

Managing by weight

A spring scale is used between the hoist and the test block to reveal the weight of this first-stage (top layer) trickling filter biomass. A new block (dry) weighs 25 pounds. With active biomass growth, the Corning team has seen the test block reach close to 140 pounds, and the growth weight likely would have continued higher. After experimenting with various flushing procedures, they established a flushing program that maintains the block regularly at under 100 pounds.

The first graph bar (Chart A, Week 1) represents the new block with harness, wet with wastewater but no biofilm. It registered under 30 pounds. Allowed to grow without interference, the test block climbed past 130 pounds by Week 15.

To release the weight of the excessive biomass, and to test the adherence of the biomass to the plastic media, the staff took a fire hose to the test block as it rested in the trickling filter bed (Chart A, Week 16). With repeated weighings, they found that only a forceful stream would dislodge the film growth.

Though this process reduced the weight, it caused significant wear and tear on the plastic modular skin, including some broken seams and folded edges. Again left undisturbed, the test block biofilm grew back at almost double the startup rate, reaching 110 pounds in five weeks (Chart A, Week 22).

Since the trickling filter was not designed with recycling, the team experimented with options for increasing the flow to the filter. Only a heavy flow, along with slowing the distributor arm, effectively sloughed down the biomass. The team proved this by capitalizing on wet-weather events as the source for increased flow.

The heavy zoogleal mass that sloughed off during wet-weather events followed the standard path from trickling filter effluent through the SRBC tanks and settling out in the final clarifiers, after which it was pumped to the primary tanks to resettle.

In trying to short-circuit the solids settling process, they found that opening the two 8-inch tank drain valves at the head of the SRBCs captured a good portion of the settleable solids coming out the trickling filter (allowing them to settle directly in the primary tanks) while the recycled flow increased the stream at the head of the plant enough to effectively flush the trickling filter. By August 2002 (Chart B, Week 13), the team had a system that worked well.

The temperature of the wastewater was also tracked, as fixed-film processes typically bulk up and slim down as the biological entities’ metabolisms shift with the seasons. But as operators took control of the feed and flush settings and events, temperature influence became insignificant.

Once a week, the motorized trickling filter arm is turned down to its minimum speed (5 Hz), and the SRBC drain valves are opened for one hour of flushing. Bacteria and advanced organisms (such as fly larvae) slough down in the first few minutes of flushing, and the Imhoff cone settleable solids readings jump from typical 2 to 5 mL/L to 175 to 200 mL/L.

Today, trickling filter BOD reduction remains constant, and occasional odor problems have been eliminated with operator control and response based on routine media observation and weighing.

Consistent results

Corning operators now realize highly consistent treatment conditions in the trickling filter and expect the plastic media to reach or exceed their 20-year life expectancy. The more operators understand how fixed biofilms grow and respond to stimuli, the better they can maintain treatment efficiency.

The principle espoused by the Corning team is that fixed-film systems can and should be biologically operated — not just “run.”

About the authors

Alan Bontorno is chief operator and Richard Johnson is a certified operator at the City Corning (N.Y.) Wastewater Treatment Plant. Both are State of New York certified Grade 3 operators. They can be reached at 607/962-2215.


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