Particulate-biofilm, expanded-bed technology for high-rate, low-cost wastewater treatment: Nitrification

Abstract

The performance of a particulate-biofilm, expanded-bed process for nitrification of activated sludge final effluent (ASFE) is reported for a plant receiving mixed industrial and domestic wastewater.

The support material for the particulate-biofilms was glassy coke, to which the nitrifying bacteria attached and formed a highly active biofilm. An average nitrification rate of 1.7±0.6 kg m⁻³_{expanded bed} d⁻¹ was recorded during operation of the bioreactor, which had a hydraulic residence time of 15 min. On average, the ASFE contained 12.6±3.7 g m⁻³ NH₃–N, which was reduced to 2.6±3.3 g m⁻³ NH₃–N. Furthermore, transfer of 10–12% of the oxygen in air was achieved using counter-current aeration.

This investigation has demonstrated that a high rate of nitrification can be achieved with a particulate-biofilm, expanded-bed process. It has also demonstrated that the process can operate without backwashing and still remove particulate material from the ASFE feed.

Introduction

Legislation has been enacted in many countries to reduce the ammonia load on receiving waters. For example, the European Urban Wastewater Treatment Directive (CEC, 1991) led the Environment Agency to impose limits for ammonia discharge from sewage and other wastewater treatment works in England and Wales. These new limits are designed to raise the quality of receiving waters so that, for example, the River Ecosystem Class (NRA, 1994) is improved and fish populations can recover. The most cost-effective way to reduce ammonia discharges is to use biological nitrification.

In nitrification, nitrosifying-bacteria (e.g. Nitrosomonas) oxidise ammonia to nitrite and nitrifying-bacteria (e.g. Nitrobacter) oxidise the nitrite to nitrate. These bacteria obtain biochemical energy from the oxidation step, and some of this energy is used to reduce carbon dioxide to organic carbon, for incorporation into biomass. Aside from requiring a reduced nitrogen species and CO₂, they also require similar nutrients to other organisms, including molecular oxygen.

In wastewater treatment, nitrifying bacteria normally have to compete for oxygen with the heterotrophic microbes responsible for BOD oxidation. For example, in fixed biofilm systems such as trickling filters, when the BOD concentration is >20 g m⁻³, nitrification is limited by oxygen availability (Harremoes, 1982). It is for this reason that a separate unit operation, following a BOD reduction process such as activated sludge, is the most effective way to achieve high-rate nitrification. If nitrification is to be achieved in the same reactor as BOD reduction, then the different microbiological processes must be separated in time (e.g. extended-aeration activated sludge or sequencing batch reactor). In other words, BOD reduction dominates initially, with nitrification proceeding once most of the organic carbon has been oxidised. Obviously, using a dual-purpose reactor requires a longer residence time than the sequential use of separate unit operations.

Nitrifying bacteria are slow growing (under optimum conditions $T_{\mathrm{d}}=8\mathrm{h}$ for Nitrosomonas, 10 h for Nitrobacter (Bock et al., 1986)) and are therefore easily washed out of conventional suspension culture systems, such as activated sludge, where the prevailing conditions often result in doubling times of 1–3 days. Thus, to operate a high-rate nitrification process, some form of biomass retention is required. Although biomass retention is the chief operational characteristic of traditional trickling filters, a high cell concentration cannot be achieved because of the large, inactive volume occupied by the biomass support material. Trickling filters typically provide a surface area for biofilm development of up to 300 m² m⁻³ and can retain a biomass concentration in the range 0.2–0.4 kg m⁻³. In contrast, activated sludge processes operate with 2–4 kg m⁻³; and recirculating bed, particulate-biofilm systems (Nicolella et al., 2000) can retain 15–30 kg m⁻³. A static bed of 1 mm particles expanded by 50% provides a specific surface area of 2400 m² m⁻³ (Fig. 1); which, theoretically, allows immobilised biomass concentrations to reach 42 kg m⁻³ (dry weight). This calculated biomass hold-up assumes a 500 μm thick biofilm of 80% water content, 60–80% packing of the cells in the biofilm, and 60% packing density of the particles.

Because the process was operated on activated sludge final effluent (ASFE) at a wastewater treatment works, it was not possible to control the chemical, physical or biological composition of our process feed. Therefore, it was not possible to perform controlled experiments and determine accurate process dynamics and kinetics for modelling purposes. It is therefore not possible to develop a universal approach to interpretation of the information at this stage in the development of our expanded bed process.