Wastewater is applied to the surface of intermittent and recirculating sand bioreactors in doses. The dosing frequency has a tremendous influence on the performance of each sand bioreactor. After each dose of wastewater, air is naturally drawn into the bioreactor, maintaining an aerobic treatment environment. Do not use a gravity distribution system that results in a trickle flow onto the bioreactor surface. The constant application of wastewater will result in premature clogging.
For intermittent sand bioreactors, the simplest dosing strategy of once per day is also the poorest. It has been known for decades that by simply dividing the daily waste flow into two equal doses per day, the treatment efficiency can be greatly increased (see Figure 8). Small, frequent doses of wastewater spaced throughout the day is one objective of a wastewater application system.
Recirculating sand bioreactors require a different dosing strategy with more frequent doses. This is a result of the increased amount of water that must recirculate through the system. Recirculation ratios typically range from 3:1 to 5:1. As a result, the amount of water applied to the bioreactor surface is increased three to five times. Once the total amount of water to be applied to the bioreactor is established, a simple dosing pattern is developed. The following calculations provide a typical dosing strategy for recirculating sand bioreactors.
Average daily flow: 10,000 gallons per day (gpd)
Recirculation ratio: 5:1
Dosing frequency: 1 dose per hour
10,000 gpd * 5 recirculations = 50,000 gpd applied to the bioreactor
1 dose per hour * 24 hours per day = 24 doses per day
50,000 gpd / 24 doses per day = 2,083 gallons per dose
The dosing strategy is based on an average daily flow into the treatment system, not the design flow. Therefore, adjustments to the dosing system must be made when influent flows change substantially.
The following techniques could be used to divide the daily load into several small doses. Remember, the objective is to apply as many small doses per day as possible.
The most reliable way to divide the daily flow of wastewater into a large number of small doses is to accumulate the wastewater in a tank equipped with a pump and a timer. The timer turns the pump on and off several times per day. A timer can be easily adjusted by the system operator to reflect water use patterns. Float switches are used to override the timer during low and high flow periods. The timer should be set to turn on and off as many times as possible with a minimum of once every four hours.
For some small systems (less than 1,000 gpd), a timer may not be practical. A submersible sewage pump controlled by float switches in a dosing chamber can be used to deliver the accumulated wastewater to the bioreactor surface in small doses. A predetermined amount of wastewater is delivered to the bioreactor surface based on a flow-weighted proportion. Set the float switches so that one pump cycle pumps 1/6 to 1/24 of the daily design flow.
Some Ohioans, such as the Amish, do not use electricity and some bioreactor systems may be considered for remote locations that do not have electrical service. To meet this special need, consider dosing a sand bioreactor with a tipping pan as shown in Figure 9. A tipping pan allows a predetermined amount of wastewater to be applied to the sand bioreactor based on a flow-weighted proportion.
Short circuiting along the edge of the bioreactor is another concern. Keep applications near the center of the sand and keep the sand surface level. A single large diameter pipe, along the center of the bioreactor, with three holes widely spaced, is one approach. An elbow at the end of a pipe, directed onto a concrete splash plate, is also effective. Another configuration is to bring the pipe up through the center of the bioreactor and surround it with a splash plate. The splash plate prevents erosion of the sand by the running water, and distributes the wastewater radially. Sprinklers that evenly distribute the wastewater in a fine spray over the bioreactor surface are also used, but an operator must make frequent inspections and correct clogged sprinklers. Examples of application systems are shown in Figure 10.
