Challenges during start-up of urine nitrification in an MBBR

Sammanfattning: Nitrogen and phosphorous compounds are secreted by humans and treated in wastewater treatments plants. These substances are also found in agricultural fertilisers and needed for plant growth. Nutrient recycling is limited in conventional treatment systems and valuable products are lost. Urine contains most of the nutrients from humans and by separating it from wastewater it is possible to close the cycle and use as fertiliser. The overall load to existing treatment plants is simultaneously decreased which is beneficial in growing urban areas. Urea in urine is hydrolysed to ammonia during storage. To prevent nitrogen loss due to ammonia volatilisation, separated urine needs stabilisation. One method is biological nitrification. Ammonia oxidising bacteria convert ammonia to nitrite while pH drops. Nitrite is then further oxidised to nitrate by nitrite oxidising bacteria. Limitations in alkalinity allow half of the ammonia to oxidise. The remaining half is stabilised in non-volatile ammonium when pH decrease. The treated solution contains equal parts ammonium and nitrate which are widely used in nitrogen fertilisers. However, the treated urine needs further processing to concentrate the solution to compete with existing products. Sege Park in Malmö, Sweden, is a housing area which aims to be an example of sustainable city development by 2025. The idea is to have one house with source separation of urine and facilities for further processing. The regional water and sewage organisation, VA SYD, therefore needs to investigate and determine an appropriate method for urine treatment. This project aimed to provide knowledge of nitrification as a stabilisation method. The start-up of a nitrification reactor for source-separated urine was studied in a bench-scale moving bed biofilm reactor, operated for two periods of 103 and 100 days respectively. The first part experienced continuous instabilities with fluctuating pH and repeated nitrite accumulations. Another start-up was initiated in the second half of the project with overall successful results. A shorter period of instabilities caused accumulation of nitrite twice at an influent nitrogen concentration of 1,390 mgN L-1. The problems were overcome by lowering the load and then by exchanging the influent pump from a fixed-flow pump to pH-regulation at pH 6.2. The urine concentration could be further increased to 4,680 mgN L-1 nitrogen in the reactor by the end of the experimental period. The corresponding nitrification rate was 0.3 gN m-2d-1 (60 gN m-3d-1). The rate decreased while the nitrogen concentration increased. Maximum rate was 0.9 gN m-2d-1 (160 gN m-3d-1) when the reactor concentration was 2,230 mgN L-1. It seems crucial to observe and counteract process instabilities early for successful long-term operation of highly concentrated nitrification reactors. Continuous monitoring of pH and dissolved oxygen in combination with nitrite samples facilitate detection of instabilities. Reactor regulation with pH controlled influent ensure ideal conditions for well-balanced bacterial interplay and thus enhanced reactor stability.