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The Nibbler

Al Hill
President,
Wastewater Eng., Inc.,
PO Box 4994,
Spartanburg, SC 29305-4994,
(864) 585-1426


Secondary Wastewater Treatment

Description

The NibblerTM is an aerobic treatment device consisting of one or more tanks each of which is equipped with air pumps. The air pumps are driven by an external regenerative air blower and function as the wastewater air contactor and the liquor mixing mechanism within the system. Each Nibbler unit also contains buoyant media which is retained by a grid below the liquid level throughout the entire surface of the tank.

Type of System

The Nibbler is designed to treat high-strength wastewater to at least residential sewage septic tank effluent quality and will reduce wastewater constituent (BOD%, TSS and Grease and Oils) to or below those levels customarily found in residential sewage septic tank effluent.

Design

Buoyant Media and Retention Grid-

Each Nibbler unit contains a specific amount of buoyant media which is critical to the successful operation of the unit. This media has a large surface area to volume ratio (38ft2/ft3). The buoyant media is retained below the operating liquid level by a retention grid made out of one inch PVC pipe ribs and polyethylene fence mesh.

Aeration System-

The Nibbler’s aeration system consists of a regenerative air blower(s) which provides the required air flow to drive the air pumps. The air blower is the only mechanical part of this system. The air pumps produce a continuous upflow of air-liquor mixture resulting in constant mixing of liquor in the tank(s) and in substantial diffusion of oxygen to the liquor by creating a large air-to-liquor contact surface area necessary for effluent diffusion of gases out of the liquor.

Control box and Metering Devices-

Each Nibbler system requires the installation of a control box and a metering device. The most common method of metering influent employs a submersible pump, one or more float switches, cycle counters and time meters, all connected and operated through a control box. The submersible pump and its float switches are located in a pump tank(s) or a surge tank(s). The float switches are used to activate the submersible pump when the liquid in the pump tank rises to a certain level. When the Nibbler system is operated basically on a demand dosing mode, there is no need for timers in the control box, as the float switch acts as the on-off mechanism. When a surge tank is part of the Nibbler system, several float switch-timer combinations may be used, each activating at a given liquid level in the tank. The lower float switch-timer operates at a lower influent injection frequency than a higher float switch-timer device, allowing for more frequent influent injections with rising liquid levels in the surge tank (i.e. higher demand). When a particular float switch is on, it activates its timer to inject one cycle with programmable “time-on” and “time-off” modes. In either case, the pump is calibrated for wastewater volume pumped per cycle and the cycle counter for each float switch-time device in the control box keeps a running count of cycles pumped.

Therefore, the volume of wastewater processed by the Nibbler in a given period can be determined by multiplying the number of cycles in that period by the number of gallons per cycle.

System Process

Mixing-

The operational flow rates of the air pumps in the Nibbler, the vertical direction of the liquor flow, and its turbulent nature combined, result in a continuous and rapid mixing throughout the entire tank(s). As depth increases below the air pumps, turbulence decreases, hence mixing decreases. Exceptions to the rule of complete mixing are evident when settleable solids (sludge) is present at the bottom of the tank(s). The density difference between the liquor and the sludge impedes mixing and the sludge is usually anaerobic. Air flow through the air pump can be regulated to increase or decrease liquor flow to maximize or minimize mixing and oxygen diffusion to the liquor.

Oxygen Diffusion-

The dissolved oxygen concentration of the liquor in an operating system is normally well below its saturation level. The amount of surface area for air-liquor contact generated in the air lift pump, the air-liquor contact time through the air-pump and the splashing at the surface of the tank provide for diffusion of gases in to and out of the liquor. The efficiency of oxygen diffusion will depend upon the difference between the actual dissolved oxygen concentration of the liquor and the oxygen saturation level for that particular system’s liquor temperature and composition. The amount of oxygen diffusion can be regulated by increasing or decreasing air flow through the air pumps.

Microbial Oxygen Uptake-

By virtue of its design, oxygen diffusion in the Nibbler occurs in the upper levels of the tank (air pumps and air-liquor interface). The direction of the liquor flow in the tank is downwards. Microbial oxygen uptake occurs at every point in the liquor’s downward path. Therefore, in an active Nibbler system, higher dissolved oxygen concentrations are observed at the top of the tank and dissolved oxygen concentration decreases with increasing liquor depth. The dissolved oxygen concentration at any point in the Nibbler tank is dynamic and will depend upon the liquor’s temperature, influent waste strength and volume, frequency of influent injection and time elapsed between influent injections.

Dissolved Oxygen Profile: The Facultative Capability-

Because dissolved oxygen concentration decreases with increasing liquor depth, the Nibbler exhibits three different zones with relation to dissolved oxygen concentration. The uppermost mixing zone, which employs most of the tank’s volume, is the aerobic zone. The transition between the aerobic and the anaerobic zones (zone of least mixing) constitutes the facultative zone. The lowest portion of the tank, where solid settle (sludge), constitutes the anaerobic zone (zone of no mixing). The actual formation of these three zones in any system depends upon the system’s age, hydraulic and waste strength loading, and mixing and oxygen diffusion characteristics of the system. A system operating near its maximum capacity will contain these three zones once sludge begins to accumulate at the bottom of the tank. An underloaded system will take longer to accumulate sludge and therefore, longer to form these three zones.

Nibbler Design Parameters

Influent to Nibbler system: 2,000 gallons per day 500 BOD5 mg/L strength

Design criteria for pre-treatment

prior to discharge: Max. BOD5 ó 230mg/L
Max. TSS ó 150 mg/L
Max. O&G ó 25 mg/L


Organic Design: Pounds Per Day Method

2,00 GPD x 500 mg/L BOD5 x .00000834 = 8.34 pounds per day

Each Nibbler Pod Section will effectively treat 0.81 pounds per day

8.34 lbs/day divided by 0.81 lbs/day/pod = 10 pods


The design of this system will use Organic Loading parameters which dictate a total of 10 Nibbler Pod Sections distributed within a 3,300 gal concrete tank.

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