Bjørn Rusten

Nitrification in a moving bed biofilm reactor

Abstract A new biofilm reactor, the moving bed reactor, was studied for nitrification purposes. The study was partly on laboratory-scale with a prepared water, and partly on pilot-scale with primary or secondary effluent as feed water. The experimental results showed that when alkalinity was in excess and there was no organic load, either the ammonium or the oxygen concentration would be limiting for the nitrification rate. The shift from the ammonium to the oxygen concentration being rate limiting occurred for an oxygen to ammonium concentration ratio of about 3 g O 2 (g NH 4 -N) −1 . The oxygen concentration had a great influence on the nitrification rate when oxygen was rate limiting. The nitrification rate was then close to a first-order function of the oxygen concentration, indicating liquid film diffusion to be the important rate limiting mechanism. Nitrification rates were reduced by increased organic loads. When the organic load exceeded 5 g total BOD 7 m 2 d −1 , the nitrification became insignificant. With a secondary effluent feed, nitrification rates of 0.7−1.0 g NO x -N (NO 3 -N + NO 2 -N) m −2 d −1 were achieved at oxygen concentrations between 4.5 and 5 g O 2 m −3 . Curves were constructed for nitrification at different organic loads when the oxygen concentration was rate limiting.

Anaerobic co-digestion of microalgae Chlorella sp. and waste activated sludge.

The study investigated the growth characteristics of environmental algal strain, Chlorella, in the modified Zarrouk medium and its anaerobic co-digestion with waste activated sludge (WAS). Analysis of extracellular polymeric substances (EPS) in algal culture and WAS indicated that Chlorella secreted more EPS into the surrounding liquid than formed floc-associated EPS as in activated sludge. Mesophilic anaerobic digestion of algae alone required extended digestion period to produce methane, with biogas yield at 262 mL/gVSfed after 45 days of digestion. When algae was co-digested with varying amounts of WAS, 59-96% in mass, not only biogas yield of microalgae improved but the gas phase was reached more quickly. The dewaterability of co-digestion products were also better than two controls digesting WAS or algae only. These results suggest that anaerobic co-digestion of algae and sludge improves the digestibility of microalgae and could also bring synergistic effects on the dewaterability of digested products for existing anaerobic digesters.

Utilisation of wastewater nutrients for microalgae growth for anaerobic co-digestion.

The feasibility of growing microalgae in natural light using wastewater high in nutrients (N & P) for the production of more bioenergy was examined. The main retrofitting unit would be a photobioreactor for wastewater treatment plants (wwtp) having anaerobic digesters in close proximity. Theoretical microalgae production rates from different wastewater sources (municipal wwtp, source separation of human and animal wastewaters) were estimated using mass balance. Mass and energy balances for a conventional wwtp using chemically enhanced primary treatment was investigated for microalgae growth for a situation limited by availability of carbon dioxide (CO2) generated onsite and where additional CO2 was imported from outside source. Reject water from dewatering of anaerobically digested sludge from four wwtp around Oslo region were pretreated for improved light penetration and examined for microalgae growth. Several pre-treatment methods were investigated. Pretreatment using flocculation + settling + anthracite filtration yielded high light transmittance. A maximum microalgae growth rate of 13 g TSS/m(2)-d was achieved using this pretreated reject water. The challenges of integrating photobioreactors with existing units have been highlighted.

Increasing the sludge energy potential of wastewater treatment plants by introducing fine mesh sieves for primary treatment

The objective of this study was to compare some basic characteristics of sludge from fine mesh sieves (sieve sludge) with sludge from primary clarifiers (primary sludge) regarding their energy potential with a focus on anaerobic digestion and/or incineration. Nineteen samples of sludge from fine mesh sieve plants (most of them without fine screens and grit chambers as pre-treatment) and 10 samples of primary sludge were analysed for the content of dry solids (DS), volatile solids (VS), chemical oxygen demand (COD), calorific value and methane potential. The results demonstrated that the sieve sludges have significantly higher VS content and higher methane potential than primary sludges, clearly indicating an increased sludge energy potential if fine mesh sieves are used for primary treatment instead of primary clarifiers at wastewater treatment plants with anaerobic digesters. If the sludges from primary treatment are to be incinerated or used as fuel in cement kilns, there is no significant difference in energy potential (given as calorific values) for the two types of primary treatment.

Upgrading a Primary Treatment Plant for Nutrient Removal

Different strategies for upgrading the Sandefjord primary treatment plant for nutrient removal have been evaluated. This plant is characterized by dilute wastewater, large flow variations and low wastewater temperatures. Phosphorus removal was evaluated based on full scale tests at the plant. Nitrogen removal in submerged biological filters was evaluated based on literature data. Cost figures indicate that sewer rehabilitation, in order to reduce the flow and increase the influent wastewater concentrations, will be a good investment.

Start-feeding of Atlantic salmon in a water recycling plant

A water recycling plant for Atlantic salmon smolt production has been developed and tested in pilot scale. The plant consists of a micro-strainer with a sieve opening of 0·063 mm, followed by two aerated submerged biological filters. The recycled water is then pumped through a dual media sand filter and a disinfection unit. Make-up water is added ahead of a heat exchanger, before the water flows through packed column aerators and back to the fish tanks. CaCO3 slurry is used for pH-control. The plant is equipped for continuous monitoring of temperature, pH, dissolved oxygen and turbidity. Tests with start-feeding of Atlantic salmon showed daily growth of 3·8–4·0% at 13–14°C and 0·8–1·0 g feed/g fish growth. The recycled water was of good quality, with median concentrations of 0·28 mg litre−1 NH4+N, 0·06 mg litre−1 NO2−N, 0·6 mg litre−1 SS and 8 mg litre−1 COD. The lowest consumption of make-up water was 2% of the flow or 25% of the total volume of the system per day. Maximum biomass density in the fish tanks was about 20 kg m−3 of water.