Wastewater Attached Growth Process

Trickling Filters

• Return activated recirculation is similar to RAs (return activated sludge).

• Sludge goes through a spray mechanism and then to a secondary clarifier, which is a sedimentation process, resulting in sloughing.

Elements of Trickling Filter

1. Rotary distribution mechanism:

   • Distributes wastewater from the top of the filter, percolating it through the interstices of the media.

2. Oxygen: Supplied by the atmosphere and circulates through the filter by natural draft, which is needed by microorganisms.

3. Microorganisms:

   • Consist of aerobic, anaerobic, and facultative bacteria (predominant), fungi, algae, and protozoans.

   • Bacteria:

     • Most common species: Achromobacter, Flavobacterium, Pseudomonas, and Alcaligenes.

     • Slime layer: Filamentous forms like Sphaerolitus natans and Beggiatoa.

     • Lower reaches: Nitrosomonas and Nitrobacter.

   • Fungi:

     • Important in industrial wastewater.

     • Common species: Fusarium, Mucor, Penicillium, Geotrichum, Sporotrichum.

     • Often responsible for clogging filters and preventing ventilation due to rapid growth.

   • Algae:

     • Only found in upper reaches of the filter where there is sunlight.

     • Provide O_2 during the daytime.

     • Common species: Phormidium, Chlorella, and Ulothrix.

   • Protozoans:

     • Responsible for keeping the bacterial population in check.

     • Predominant species include Vorticella, Opercularia, and Epistylis.

Types of Media

4. Filter Media:

   • Rock and Gravel

   • Fiber Mesh Pads

     • Characteristics:

       • High Specific Surface Area

       • High Void Space

       • Lightweight

       • Biological Inertness

       • Chemical Resistance

       • Mechanical Durability

       • Low Cost

     • Thin fibers similar to air conditioning filters but formed into heavier and thicker pads.

   • Brillo Pads

     • Similar to the mesh pad, also called the "ribbon bundle" type packing.

   • Random or Dumped Packings

     • Injection molded plastic shapes.

     • Must be installed over a grid or screen-type support.

     • Made from PP (polypropylene) or HDPE (high-density polyethylene); also available in stainless steel, ceramic, porcelain.

   • Structured Packings

     • Have virtually all characteristics one looks for in the "ideal" packing.

     • Typically constructed of vacuum-formed sheets of PVC (polyvinyl chloride).

     • Lower cost per unit surface area than injection-molded packings.

   • Precast blocks of vitrified clay or fiberglass for rock media.

   • Precast concrete beams supported by posts for plastic media.

Media Evaluation

A table provides a scoring/ranking system for trickling filters based on packing type, with the following criteria:

• Surface area

• Void fraction

• Free pass diameter

• Plugging potential

• Material of construction

• Strength

• Flexibility

• Maintenance

• Energy consumption

The ratings are on a scale of 1 to 5, where 1 is the worst and 5 is the best; 'A' denotes acceptable:

Underdrains

• Support the filter medium.

• Remove biological "flocs".

• Collect the treated effluent and the sloughed biological solids.

• Material of construction.

Process Flow Diagram

• Influent -> Nutrient Addition (N and/or P) -> Trickling Filter -> Secondary Clarifier -> Sludge Pump -> Waste Sludge -> Recycle Pump -> Recycle.

Objectives of Trickling Filter

• Reduce strength of filter influent.

• Maintain constant wetting rate.

• Force sloughing to occur, increase shear forces.

• Dilute toxic wastes.

• Reduce the nuisances of odor and flies.

• Increase air flow.

Trickling Filter Rate Comparison

Rotating Biological Contactor (RBC)

• A variation of the attached growth idea provided by the trickling filter.

• Relies on microorganisms that grow on the surface of a medium; it's a fixed film biological treatment device.

• The basic biological process is similar to that occurring in trickling filters.

• An RBC consists of a series of closely spaced (mounted side-by-side), circular, plastic, synthetic disks, typically about 11.5 ft in diameter.

• Attached to a rotating horizontal shaft; approximately 40% of each disk is submerged in a tank that contains the wastewater to be treated.

• As the RBC rotates, the attached biomass film (zoogleal slime) that grows on the surface of the disks moves into and out of the wastewater.

• While submerged, the microorganisms absorb organics; while rotated out, they are supplied with needed oxygen for aerobic decomposition.

• As the zoogleal slime re-enters the wastewater, excess solid and waste products are stripped off the media as sloughings.

• These sloughings are transported with the wastewater flow to a settling tank for removal.

RBC Process Flow

Raw -> Primary Settling Tanks -> RBC -> Secondary Settling Tanks -> Cl2 -> Effluent -> Solids Disposal.

Limitations

• Organic and hydraulic shock loads.

• Process efficiency decreases during colder temperatures.

• Lack of operational flexibility.

Advantages

• Process simplicity and stability.

• Very low maintenance cost, largely limited to greasing of bearings and inspecting chains/sprockets for wear and slack.

• Low disc speed achieves sufficient mixing and aeration while consuming relatively little power.

Hydraulic Loading Rate

• The manufacturer normally specifies the RBC media surface area, and the hydraulic loading rate is based on the media surface area, usually in square feet (ft^2).

• Hydraulic loading is expressed in terms of gallons of flow per day per square foot of media.

• Helpful in evaluating the current operating status of the RBC; comparison with design specifications can determine if the unit is hydraulically over or underloaded.

• Hydraulic loading on an RBC can range from 1 to 3 gpd/ft^2.

Example 9

An RBC treats a primary effluent flow rate of 0.244 MGD. What is the hydraulic loading rate in gpd/ft^2 if the media surface area is 92,600 ft^2?

\frac{244,000 \frac{gal}{day}}{92,600 ft^2} = 2.634 \frac{gal}{day \cdot ft^2}

Example 10

An RBC treats a flow of 3.5 MGD. The manufacturer's data indicate a media surface area of 750,000 sq ft. What is the hydraulic loading rate on the RBC?

\frac{3,500,000 \frac{gal}{day}}{750,000 ft^2} = 4.67 \frac{gal}{day \cdot ft^2}

RBC Process Control Calculations

Soluble BOD

• The soluble BOD concentration of the RBC influent can be determined experimentally in the laboratory, or it can be estimated using the suspended solids concentration and the K factor.

• The K factor approximates the BOD (particulate BOD) contributed by the suspended matter.

• The K factor must be provided or determined experimentally in the laboratory.

• The K factor for domestic wastes is normally in the range of 0.5 to 0.7.

Organic Loading Rate

• The organic loading rate can be expressed as total BOD loading in pounds per day per 1000 square feet of media.

• Actual values can be compared with plant design specifications to determine the current operating condition of the system.

Example 11

An RBC has a media surface area of 500,000 sq ft and receives a flow of 1,000,000 gpd. If the soluble BOD concentration of the primary effluent is 160 mg/L, what is the organic loading on the RBC in lb/day/1000 sq ft?

\frac{1,000,000 \frac{gal}{day} \times 8.34 \frac{lbs}{gal} \times 160 \frac{mg}{L}}{500,000 ft^2} \times 1000 = 2.67 \frac{lb}{ \text{day} \cdot 1000 ft^2}

Example 12

The wastewater flow to an RBC is 3,000,000 gpd. The wastewater has a soluble BOD concentration of 120 mg/L. The RBC consists of six shafts (each 110,000 sq ft), with two shafts comprising the first stage of the system. What is the organic loading rate in lb/d/1000 sq ft on the first stage of the system?

[The solution is not provided in the text but the calculation would follow a similar structure to Example 11, using the flow rate, BOD concentration, and surface area of the first stage (2 shafts x 110,000 sq ft = 220,000 sq ft).]