Two types of gravity systems are available: (1) settling Lagoons, which provide retention time for solid particles to settle as sludge. These must be cleaned periodically; and (2) mechanical Clarifiers, which remove suspended solids and also rid bottom sludges mechanically.
The settling lagoon requires a minimum capital investment. Cleanout costs are high compared with the mechanical clarifier operating costs.
Settling devices are usually designed on the basis of overflow rate, gal per day (gpd) per sq ft of surface area. According to the Ten State Standards (Great Lakes-Upper Mississippi River Board of State Sanitary Engineers 1968), this rate should be in the range of 600 to 1000 gpd/sq ft. In designing the carbon settling lagoon, frequency of lagoon cleaning must be considered, and the lagoon must be sized accordingly. Carbon sludge will settle to a density of 5-20% solids.
TABLE 8.1.1 ALGAE REMOVAL PROCESSES: MERITS AND FLAWS
Process Advantages Limitations
Coagulation and settling Sand filters
Simple and inexpensive Simple and inexpensive Positive removal of all types of algae Positive removal of all types of algae Simple and inexpensive Positive removal of all types of algae
Creates toxicity; only some algal forms attacked High doses needed; not all algae attacked High chemical doses needed; difficult sludges produced Rapid filter clogging may occur
Not all algal forms removed
Not all algal forms removed; sludges may be difficult to handle
TABLE 8.1.2 EXAMPLE: SETTLING LAGOON FILL TIME CALCULATION
Settling Lagoon Data:
Flow = 10 million gal/day (mgd) Influent concentration = 1000 mg/l Effluent concentration = 50 mg/l Sludge density = 5%
Carbon deposited per day:
Solids capacity of lagoon at 5% sludge density:
Capacity = 0.42 — X 8.3 X 106 gal = 3.5 X 106 lb solids gal
Time required to fill lagoon with sludge: 3.5 X 106 lb 8 X 104 lb/day
As an example, a 5-acre lagoon, 5 ft deep, with an influent suspended solids concentration of 1000 mg/l and an effluent concentration of 50 mg/l at a flowrate of 10 mgd will retain almost 80,000 lb of solids per day. If the solids settle to a 5% sludge density, the lagoon will be filled with sludge in less than two months, as indicated by the calculations in Table 8.1.2. A settling lagoon design for this application would probably be based on cleaning frequency rather than on overflow rates.
The outfall structure of a settling system should retain floating material and maintain laminar flow to prevent solids from resuspending at discharge due to turbulence. An underflow-overflow weir (Figure 8.1.1) efficiently provides such an outfall. According to the Ten State Standards (Great Lakes-Upper Mississippi 1968), weir loading rates should not exceed 10,000 gpd per linear ft of weir to assume minimum resuspension of settled matter from turbulent flow. For the example in Table 8.1.2, a weir 1000 ft long would be required.
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