Moving beds - a trickling filter approach
HV’s Senior Water Technology Expert, Ir. Arnold Zilverentant outlines a new methodology, developed in the Netherlands, for combined gas and water treatment which also manages to control biomass growth.Many industries produce both wastewater and odorous gas streams. It is not uncommon to treat small volumes of gas in an aerobic wastewater treatment plant, thus reducing the odour nuisance without additional investment costs for gas treatment.
The Moving Bed Trickling Filter (MBTF) is a newly developed concept making an adequate control of the biomass growth possible. Furthermore, the relatively low-pressure loss yields a cost efficient treatment of the combined wastewater and large gas flow in one compact reactor.
Typical for the MBTF is the option for (periodical) transportation of not only water and gas but also the specially designed carrier through the reactor. Biomass sloughing is achieved by mechanical treatment via circulation of the carrier and subsequent removal of the biomass through a separate discharge. This system enables control of the biomass growth and a high loading rate can be applied without clogging problems. Therefore, the MBTF is compact and has a very small footprint.
Proven on full-scale
Currently three full-scale units are in use in the Netherlands, all achieving high removal efficiencies. The full-scale results show that the unique carrier combines a high specific surface with a high porosity, a high liquid retention and a high resistance to abrasion. Two carrier diameters are currently available: 18 and 26 mm, providing 420 respectively 320 m²/m³ of specific surface.
Varying the flow of carrier material over the reactor proved to be an excellent control of the biomass growth. The specific sludge production (kgDS/kgCOD removed) increases with decreased carrier recirculation time (CRT). When operating in full-scale, the complete carrier bed can be cleaned in approximately 10 hours, but for optimal COD removal the CRT should be at least 2.5 days. The specific sludge production at a CRT of 2.5 days is 0.12-0.2 (kgDS/kgCOD removed). The produced surplus sludge showed to be applicable as bio-polymer, making the use of chemical coagulants on the preceding FFU obsolete, thus reducing the operational costs for the total treatment substantially.
The relatively low pressure drop over the carrier bed enables large hydraulic loading rates to be applied. At a liquid loading rate of 6 m³/(m²h) and a gas loading rate of 1,500 Nm³/(m²h), the specific pressure loss in co-current operation is approximately 250 Pa/m.
Up to loading rates of 20 kgCOD/(m³d), a near constant removal efficiency is achieved. For normal ‘sweet’ wastewater the efficiency is 85-90%, while for saline wastewater (1.5-2.5% of chloride) the efficiency is 75-80%. Nitrogen removal efficiency is in the range of 30-60%, with indications that complete nitrification is feasible as long as the COD loading rate does not exceed 2.5-3 kg/(m³d).
On all three locations reports on odour nuisance virtually disappeared after completion of the MBTF.
Promising pilot research
In a pilot treatment of a gas flow polluted with styrene a removal efficiency of 90% was achieved at loading rates as high as 150 g/(m³h). Compared to conventional biological treatment the MBTF showed much higher efficiencies.
In another pilot research the MBTF was used as a stripping column for methane removal from ground water, containing a high level of iron. Results showed a transfer coefficient (Kla) of 0,079 sec-1, being higher then conventional pall rings. No clogging due to iron-hydroxide precipitation occurred.
Overall the MBTF has proven to be capable of treating wastewater and off-gas at high loads and with high efficiencies, both separately as well as together in one compact reactor.
Even difficult waste streams such as styrene containing off-gas, saline wastewater and iron loaded ground water were treated successfully.
Full-scale evaluation showed the ROI (Return On Investment) under Dutch conditions to be in the range of 2-4 years. While in other systems the investment in gas treatment does not provide any cost savings, the MBTF yields a profit from the investment through the savings on effluent discharge fees.