Introduction:
Water is the most important natural gift for human being and aquatic life. Due to continuous technological growth and industrialization, the water has been fully polluted and categorized as water pollution. The outlet water of industries and human which accedes the limit of organic and inorganic components in the water is called wastewater (polluted water).
Sources of Waste Water:
- Industrial
- Municipal
- Commercial
Characteristics of Waste Water:
- Physical Characteristics
- Chemical Characteristics
- Biological Characteristics
Some of them appear in dissolved form; some of suspended form and some of colloidal form are known as suspended impurities, colloidal impurities and dissolved impurities respectively. To maintain the quality of water, its need the treatment of wastewater. The basic treatments of wastewater are:
- Physical Treatment
- Chemical Treatment
- Biological Treatment (Rotating Biological Contractors)
- Advanced treatment / Tertiary Treatment
Rotating Biological Contactor:
Rotating Biological contractor is one type of treatment system. Treatment system with RBC can be designed to provide to provide secondary or advanced level of treatment. Effluent BOD characteristics for secondary treatment are comparable to well – operated ASP. Where a nitrified effluent is required, RBC can be used to provide combined treatment for BOD and ammonia nitrogen, or to provide separate nitrification of secondary effluent.
The RBC process applicable typically consists of a number of units operated in series. The number of stage depends on the treatment goal, with two to four stages of BOD removal and six or more stages for nitrification. Stages can be accomplished by using baffles in a single tank or by use of separate tanks in series. The suppliers of RBC equipment differ in there disc designs, shaft, and packing support, and configuration designs. The principal elements of a RBC system design are the shaft, disk materials and configuration, drive system, enclosures and settling tanks.
Process Selection:
Choice of the process mode most applicable will be influenced by the degree and consistency of treatment required, type of waste to be treated, site constraints, and capital and operating costs. The process design of a RBC facility involves an accurate determination of influent, septage dumps, and sidestream loadings, proper media sizing, staging and equipment selection to meet effluent requirements, air requirements, and selection of an overall plant layout that shall provide for flexibility in operation and maintenance.
In all RBC systems, the major factors controlling treatment performance are:
- Organic and hydraulic loading rates;
- Influent wastewater characteristics;
- Wastewater temperature;
- Biofilm control;
- Dissolved oxygen levels; and
- Flexibility in operation.
Basis of RBC Design:
Unit Sizing:
Organic loading is the primary design parameter for the RBC process. This is generally expressed as the organic loading per unit of media surface area per unit of time, or in units of pounds BOD5 per thousand square feet per day.
Wastewater temperatures above 55 degrees F have minimal affect on organic removal and nitrification rates; however, below 55 degrees F, manufacturers shall be contacted by the designer to obtain the various correction factors that must be utilized to determine the needed additional media surface area.
In determining design loading rates on RBC’s, the following parameters should be utilized:
1. Design flow rates and primary wastewater constituents;
2. Total influent BOD5 concentration;
3. Soluble influent BOD5 concentration;
4. Percentage of total and soluble BOD5 to be removed;
5. Wastewater temperature;
6. Primary effluent dissolved oxygen;
7. Media arrangement, number of stages and surface area of media in each stage;
8. Rotational velocity of the media;
9. Retention time within the RBC tank(s);
10. Influent soluble BOD5 to the RBC system including SBOD from in-plant sidestreams, septage dumps, etc;
11. Influent hydrogen sulfide concentrations; and
12. Peak loading, BOD5 max/BOD5 avg.; TKN max/TKN avg.
Loading Rates:
When peak to average flow ratio is 2.5 to 1.0 or less, average conditions can be considered for design purposes. For higher flow ratios, flow equalization should be considered.
The organic loading to the first stage standard density media should be in the range of 3.5 to 6.0 pounds total BOD5 per thousand square feet per day or 1.5 to 2.5 pounds soluble BOD5 per thousand square feet per day. First stage organic loadings above 6 pounds total BOD5 or 2.5 pounds soluble BOD5 per thousand square feet per day will increase the probability of developing problems such as excessive biofilm thickness, depletion of dissolved oxygen, nuisance organisms and deterioration of process performance. The most critical problem in most instances is the structural overloading of the RBC shaft(s).
For average conditions, the design loading should not exceed 2.5 pounds of soluble BOD5/1,000 square feet of standard media surface per day on the first stage shaft(s) of any treatment train. Periodic high organic loadings may require supplemental aeration in the first stage shafts. High density media should not be used for the first stage RBC’s.
For peak conditions, the design loading shall not exceed 2.0 pounds of soluble BOD5/1,000 square feet for the first high density media shaft(s) encountered after the first two shafts or rows of shafts in a treatment train.
For average conditions, the overall system loading shall not exceed 0.6 pounds of soluble BOD5/1,000 square feet of media. This soluble BOD5 loading to all shafts should be used to determined the total number of shafts required. The equation under section C.3.c could be used as an option to determine the number of stages required.
Staging Units:
Staging of RBC media is recommended to maximize removal of BOD and ammonia nitrogen (NH3-N). In secondary treatment applications, rotating biological contactors shall be designed with a minimum of three stages per flow path. For combined BOD5 and NH3-N removal, a minimum of four stages is recommended per flow path as shown below. Whenever multiple process trains are employed with three or more shafts in a row; the flow path should be introduced perpendicular to the shafts, and the wastewater should be distributed evenly across the face of the RBC’s.
The organic loading must be accurately defined by influent sampling whenever possible. For existing facilities that are to be expanded and/or rehabilitated it is unacceptable to only calculate the expected load to the shafts. Flow and load sampling must be done to demonstrate the load which is generally accomplished by composite sampling after primary clarification. To predict effluent quality for a range of loadings, the influent and effluent soluble-to-total BOD5 ratio can be assumed to be 0.5.
An alternative method of estimating soluble organic removal in the interstages, devised by E.J. Opatken, utilizes a second order reaction equation. The equation may be used for RBC design during the summer months; however, a temperature correction factor should be used used for the cold winter months. Wastewater temperatures below 15oC decrease shaft rotational speeds and increase loping problems resulting with insufficient biomass sloughing. This equation is as follows:
Cn = -1 + [square root (1 + 4kt (Cn -1)]/ 2kt
where:
Cn = is the concentration of soluble organics in the nth stage (mg/l);
k = is the second-order reaction constant of 0.083 (l/mg/hr);
t = is the average hydraulic residence time in the nth stage (hour); and
Cn-1 = is the concentration of soluble organics entering the ninth stage (mg/l).
The design engineer shall be aware that this equation shall be used only where appropriate, and that in the available RBC literature there may be a number of applicable equations.
Plant Design Considerations:
1. Enclosures
2. Flexibility and Flow Control
3. Monitoring
Equipment Considerations:
1. Shafts
2. Media
3. Drive Systems
4. Bearings
5. Load Cells (Hydraulic or Electronic
AES Arabia Ltd systems are an advanced evolution of the latest Rotating Biological Contractor (RBC) treatment methods including aerobic and anaerobic.
