Wastewater BOD / COD Removal

Acclimated Beads

Bead Filters as Biological Filters

A floating bead media filter acts as a fixed film reactor with bacteria growing on the surface of the beads in a layer that is a hundreds of microns thick. If biological activity is desired, the filter must be managed to encourage the culture of heterotrophic bacteria. Typical control issues include flowrate, substrate concentration, oxygen supply, backflush frequency, temperature, and pH. Fortunately, development of a heterotrophic fixed film is a natural process easily accomplished in an expandable granular format. All PolyGeysers used as biological filters should employ EN media which displays a better porosity and provides protection for the biofilm during high frequency backwashing events. The units display a high specific surface area for biofilm development in the range of 1100-1200 m²/m³.

Frequent Backwashing Prevents Biofouling

Bead filters can be operated in a wide variety of BOD concentration regimes. The PolyGeyser units are virtually impervious to biofouling, and with a high backwashing frequency, they can be effectively used as roughing filters reducing particulate BOD of raw wastes with high BOD and TSS concentrations. Roughing filters are managed with a high frequency of backwashing (down to about once an hour) to prevent clogging of the bed. Peak conversions on waste streams with CBOD₅ levels below 100 mg/L typically fall in the 3-6 hour backwash interval range. Rapidly growing biofilms uptake dissolved organics producing biomass that is then removed before the demand can be fully expressed reducing oxygen demand.

CBOD Conversion Rates

Conversion rates for CBOD are measured in units of kilograms per cubic meter of beads per day (kg/m³-day). In general, the removal rate is controlled by the CBOD loading rate (kg-CBOD₅/m³-day). For CBOD effluent qualities below about 30 mg/L the conversion rate is proportional to the effluent concentration. For example, for a domestic waste source, the unit’s removal rate has been observed to be 2 kg/m³-day at a target effluent concentration of 10 mg-CBOD₅/L. At higher CBOD concentrations the removal relation tends to increase exponentially probably reflecting the increasing proportion of the CBOD₅ that is associated with particulates. Removal of organic particulates greater than 30 microns is rapid. No biological activity is required, just capture and remove. Insight into the potential removal rate for any application is gained by analyzing a sample for soluble and particulate CBOD.

Recirculating Filtration

Most CBOD removal problems are addressed with filters configured to recycle water multiple times through a PolyGeyser. The need for recirculation is largely driven by the need to supply oxygen to the submerged bed. Recirculation also facilitates a longer contact time with the bacteria-laden bead media. Water is in contact for only about 20 seconds in a single pass through the bed. During this 20 seconds, a heavily loaded filter will consume most of the oxygen in the water dropping the concentration from around 5 mg/l to below 1 mg/l. Recirculating the water repeatedly through the bed can increase the oxygen supply to the 6-10 kg/m³-day range demanded by a heavily loaded filter. CBOD removal rates are typically 2-3 times the heterotrophic oxygen consumption rates. Recirculation rates are normally set at 10 gpm/ft³ but can be reduced to conserve energy whenever oxygen demands allow.

The RCPG series has a recirculation tank built into the unit to facilitate recirculation. The units employ energy efficiency using airlifts to recirculate and aerate simultaneously in a low head (typically 4-6 inches) recirculation. The HPPG series can be configured with a packed tower for an adjoining tank, however, lifts are of the order of 10-15 feet. LPPG units can be configured with airlifts and a matching tank at the same elevation. These units are ideal for treatment of organically rich waters that are relatively free of fixed suspended solids.

Total BOD (TBOD20 or TBOD₅) will be impacted by the particulate removal, by the heterotrophic action on dissolved organics, and by removal of ammonia/nitrite by nitrifying bacteria. Carbonaceous BOD (CBOD20 or CBOD₅) is impacted by the particulate and dissolved organic removal, but not the nitrifiers as their activity is suppressed in the CBOD test. All bead filters nitrify even at high organic loadings as long as oxygen is present in the bed. It must be presumed that a nitrifier seed is available in any effluent sample collected, thus inhibition (CBOD) is required to get an accurate assessment of organic reduction. This is particularly important when attempting to comply with stringent effluent standards.

Concentrating Biosolids

Sludge production and handling is an issue anytime an aerobic process is used for BOD reduction as approximately 0.6 kg of biofilm is produced per kg of BOD removed (kg/kg). As part of their backwash cycle, PolyGeyser units shed, settle, and concentrate biosolids produced by the BOD conversion process. When a high-frequency backwashing is employed biodegradation of the biosolids (sludge) is minimized and perhaps 0.5 kg/kg of solids must be removed from the filter. Assuming a sludge concentration of 1% (10,000 mg-TSS/l), about 50 liters (13 gallons) of sludge is produced for every kg of BOD that is removed. For this reason, PolyGeysers designed for heavy BOD removal loads should be equipped with automated or pneumatic sludge removal. A means of the handling, treating and disposing of the sludge should be provided. Common practice is to discharge the sludge to an upstream clarifier.