Double contact is the usual method, with triple contact being unusual. A common depth for contact beds is about 4 feet but ranges from 24 inches to 6 feet. Discharge after each filling is typically one-third of gross capacity, allowing for filtering medium and bacterial growths. Two hours may be taken to fill a bed followed by an hour's standing time; alternatively, filling can occur in one hour with two hours allowed for standing. A further five hours are needed for the effluent to drain away and air to penetrate between stones. Beds can thus be filled three times daily, handling 168 gallons per cubic yard per day under double contact process conditions. Should solids accumulate excessively, beds require cleaning or washing.
The floor and walls of beds must be watertight; total area should ensure quick filling and emptying without exceeding two hours’ dry weather flow for a single bed. In double contact systems, the second bed is placed at a level allowing easy discharge from the first. Finer material is used in the second than in the first to filter out finer particles.
Percolating filters differ from contact beds as they allow continuous liquid application and free effluent drainage through an opening at floor level. Walls need not be watertight, but honeycombed walls are undesirable due to chilling effects and fly breeding grounds. Even distribution of sewage is crucial for efficiency; automatic sprinklers or water wheels can achieve this.
Dosing chambers store sewage when flow is small and discharge it fully when full, ensuring filters work at a constant rate. Filters should be 4 feet deep with material gauges from half to one inch being normal practice. Ponding on the surface of bacteria beds may occur due to gelatinous fungoid growths; resting or chemical treatment can resolve this.
<Callout type="important" title="Critical Cleaning Schedule">Regular cleaning and washing are essential for maintaining optimal performance of contact beds, especially after extended periods of use.</Callout>
The activated sludge process involves aerating sewage to oxidize impurities before removing suspended solids. This method is more efficient than traditional approaches due to better oxygen utilization and continuous agitation.
Key Takeaways
- Double contact is the standard method for sewage treatment, with triple contact being less common.
- Contact beds should be cleaned regularly to prevent choking and maintain efficiency.
- Percolating filters require even distribution of sewage and careful grading of filter material.
Practical Tips
- Ensure that dosing chambers are properly sized to manage varying sewage flow rates effectively.
- Use finer materials in secondary contact beds for better filtration results.
Warnings & Risks
- Avoid honeycombed walls in percolating filters as they can lead to chilling and fly breeding issues.
- Regular cleaning is necessary to prevent excessive buildup of solids in contact beds.
Modern Application
While the specific methods described here may seem outdated, understanding these foundational principles remains crucial for modern sanitation practices. Techniques like double contact and percolating filters provide a basis for more advanced systems used today, such as activated sludge processes. Knowledge of historical approaches helps in troubleshooting current systems and adapting to new challenges.
Frequently Asked Questions
Q: What is the typical discharge rate after filling a contact bed?
The typical discharge rate after filling a contact bed is about one-third of its gross capacity, with two-thirds occupied by filtering medium, bacterial growths, deposits, and water that does not drain away.
Q: Why are honeycombed walls undesirable in percolating filters?
Honeycombed walls can cause the beds to become unduly chilled in cold weather and provide breeding places for flies, making them an undesirable construction method.
Q: What is the recommended depth of bacteria beds?
The recommended depth of bacteria beds ranges from 4 feet to a maximum of 10 feet, with many authorities preferring depths between 6 and 10 feet for optimal performance.