The Aldine Independent School District is located in Houston, Texas. Several of the district’s schools—Grantham Academy, Wilson Intermediate and Aldine Middle School—have independent package wastewater treatment plants, each with a separate NPDES permit for discharge to open Texas waterways. After decades of service, the systems were at the end of their service life.
The district sought competitive bids for newly constructed replacement package treatment plants that would provide complete treatment conforming to its existing discharge permits. Through improved treatment, the systems were to provide increased capacity without increased discharge. The systems also needed to accommodate unusually high loading rates. Typical domestic sewage designs accommodate biological oxygen demand (BOD) loading of 200 mg/L, but the dense population within the schools results in rates of up to 300 mg/L.
In March 2008, Layne Texas was awarded the competitive bid. Low price and the company’s experience and history in water and wastewater contributed to the award decision. The company had offered complete services as both general contractor and manufacturer.
The package treatment plants included aeration basins, digesters, mechanical clarifiers, equalization basins and chlorine basins. Manning Engineering, a subcontractor to Layne, performed the calculations for loading and design. Manning Engineering also provided PE approval of the final design.
Led by Project Manager Mike Moreno, the three systems were entirely designed and fabricated by Layne Texas , and each was customized to accommodate the unique maximum flow for each site: 42,000 gal per day (gpd) at Wilson Intermediate, 60,000 gpd at Grantham Academy and 63,000 gpd at Aldine Middle School.
The plants use conventional, continuous activated sludge processing with a complete mix mode, which accommodates higher loading and enables a smaller footprint. Each system includes automated flow metering and chemical feed.
The state expects 85% removal of BOD and total suspended solids. Actual results with this type of system are typically 95% and sometimes more. Ammonia levels are expected to be reduced from the high level of 35 to 40 mg/L to a reduced level of 3 mg/L. Under normal operating conditions, these plants are anticipated to reduce ammonia to less than 1 mg/L.
The resulting sludge is removed once concentrations reach 2% to 3% in the digester and can be either wet hauled for land application or removed to a landfill.
The design of these package treatment plants is highly adaptable, accommodating the addition of tertiary treatment filters. With tertiary treatment, these systems are capable of providing final water quality suitable for irrigation or for discharge to recreational waters.