Maximizing biogas production
and phosphorus recovery by ion exchange

 

 

Hammarby Sjöstadsverk

was 2008 jointly taken over by KTH Royal Institute of Technology and IVL Swedish Environmental Research Institute with wastewater treatment lines in pilot plant scale (150 p.e. = 1,5 m3/hour):

·                Aerobic treatment with activated sludge process and sedimentation.

·                Aerobic treatment with membrane bioreactor (MBR), an aerobic reactor with submerged micro filter, and drum filter for separation of primary sludge.

·                Anaerobic treatment with UASB-reactors (Upstream Activated Sludge Blanket).

·                The produced sludge can be thickened, digested and dewatered.

 

            

Activated sludge                                 MBR                    UASB and sludge digesters

 

Erik Levlin

40%:

Dep. of Land and Water Resources Engineering,

Royal Institute of Technology, KTH

S-100 44 Stockholm, Sweden

20%:

Hammarby Sjöstadsverk,

IVL Swedish Environmental Research Institute,
Stockholm Sweden

E-mail: levlin@kth.se

 

Keywords: pilot plant, biogas production, nutrient removal, phosphorus recovery

 

       

 

Global warming

Global warming can be counteracted by substituting fossil fuel with biogas from digesting sewage sludge, thus reducing climate impact from carbon dioxide emissions. A challenge for future development of wastewater treatment is that demands on effluent quality, especially for nitrogen removal, can come in conflict with the desire of a high production of biogas, since maximising the amount produced biogas yields less carbon source available for nitrogen removal (Levlin, 2010). During biological nitrogen removal in activated sludge, half of the organic content is oxidized to carbon dioxide and the produced sludge is less degradable for biogas production. Digestion of the organic content in the wastewater can give 80 % conversion to biogas (Kepp and Solheim, 2000). Biogas is more easily produced from primary sludge than from excess sludge from activated sludge process with biological nutrient removal. Primary sludge is easily bio-degradable since it consist of more easily digestible carbohydrates and fats, compared to excess sludge which consists of complex carbohydrates, proteins and long chain hydrocarbons (Gary et al., 2007). In the activated sludge process is half oxidized and of the half remaining in sludge is about 40 % converted to biogas, which gives that about 20 % of the organic content of the wastewater is transformed to biogas. Development of new methods for nitrogen removal without carbon source would give a large increase of biogas production from Swedish wastewater treatment.

Increased biogas production

Project at Hammarby Sjöstadsverk

At Hammarby Sjöstadsverk a project for increased biogas production from digestion of sludge has started. The idea is to dewater digested sludge and recycle it back to the digestion. By making the sludge retention time longer than the hydraulic retention time more biogas can be produced with the same reactor volume. Since methane producers grow more slowly the long sludge retention time makes the process more stable and biogas production is increased. Another idea to study is to hydrolyze the sludge through ozonation. Recycling and ozonation can give 80% sludge reduction (Yasui et al., 2005). Ozonation is a method that mostly is used for reducing sludge volume, but has also been studied for achieving increased biogas production. Ozone treatment has two counteracting effects. Partial destruction of the microbial sludge cells and degradation of undegradable molecules and cell structures will give an increased amount of dissolved organics and higher biodegradability of the sludge increases the biogas production. However, oxidation of organic material that is degradable may decrease the biogas production.

 

 

Phosphorus recovery

The phosphorus content in the digested sludge can be recovered by acid leaching and used as fertilizer in the agriculture. Experiments with leaching inorganic sludge residues with acid and base showed that the high amount of leached phosphorus and the relatively lower amount of leached metals obtained at acid leaching together with the low cost for acid makes acid leaching the most favourable and less expensive alternative (Levlin, 2006). To dissolve one mole phosphate three equivalent of acid is required independent of to that metal the phosphate is connected. The dissolved amount of metal shows that mainly metal phosphates have been dissolved and that other metal compounds have remain undissolved. However, the dissolved metals and anions from the acid have to be separated from the phosphate in a second process step. If the separation is done with ion exchange the minimum amount of chemicals for leaching and regeneration will be six mole acid and three mole base per mole phosphate. An alternative method is to mix the sludge with balls of cation exchange material. The ion exchange material will realise hydrogen ions, take up metal ions and only phosphate will be dissolved. The ion exchange material is regenerated with acid and the minimum amount of chemicals will be three mole acid per mole phosphate.

 

Anaerobic treatment without carbon source

With anaerobic treatment a very high biogas production can be achieved, however, without possibility for biological nutrient removal. Reverse osmosis for nutrient removal has been studied (Kieniewicz, 2006), but the energy consumption is very high. A process based on struvite (magnesium ammonium phosphate, MgNH4PO4) precipitation with magnesium and the anammox reaction have been proposed for nutrient removal without need of carbon source (Levlin, 2009). Half of the ammonia content in the sewage water exceeding the phosphate content is extracted as ammonia and the rest as struvite. By oxidizing ammonia in struvite to nitrate, the Text Box:  amount of struvite larger than the phosphate content can be redissolved. The nitrate of the dissolved struvite can with the extracted ammonia in an anammox process be converted to nitrogen and the magnesium and phosphate can be returned for struvite precipitation. The produced struvite can be used as fertilizer in the agriculture.

 

 

Cost estimation:

Acid leaching and separation with ion exchange. Minimum need of chemicals per mole FePO4 is 6 mole HCl (0.03 Skr/mole) and 3 mole NaOH (0.10 Skr/mole).
Cost: 2 x 3 + 10 = 16 Skr/kg P.

Minimum need of NaOH at basic leaching is 3 mole per mole FePO4 + 1 mole per mole Al.
Cost: >10 Skr/kg P.

 

Combined leaching and metal separation with use of ion exchanger.
Minimum
need of chemicals is 3 mole HCl per mole FePO4.
Cost: 3 Skr/kg P.

References

Gary, D., Morton, R., Tang, C.-C. and Horvath, R. (2007) The effect of the Microsludgeä treatment process on anaerobic digestion performance. Water Environment Federation´s Annual Technical Exhibition and Conference, San Diego USA 13-17 October 2007.

Kepp, U. and Solheim, O.E. (2000) Thermo dynamical assessment of the digestion process, 5th European Biosolids and Organic Residiuals Conf. November 2000, Cedar Court, Wakefield, UK

Kieniewicz, A. (2006) A reverse osmosis (RO) plant for sewage treatment and nutrient recovery - the influence of pre-treatment methods. Master thesis, Dept Land and Water Resources Engineering, Royal Institute of Technology KTH.

Levlin, E. (2010) Maximering av slam och biogasproduktion för att motverka global uppvärmning Vatten 66(1): 67-73.

Levlin, E. (2009) Nutrient removal without carbon source for achieving maximum biogas production and phosphorus recovery. IWA 2nd Spec. Conf. Nutrient Management in Wastewater Treatment Processes. Kraków, Poland, 6-9 Spetember 2009, 1161-1163.

Levlin, E. (2006) Phosphorus recovery with acid and base from inorganic sewage sludge residues. IWA Spec. Conf. - Sustainable sludge management. Moscow Russia, 29-31 May 2006, 612-619.

Yasui, H., Komatsu, K., Goel, R., Li, Y.Y. and Noike, T. (2005) Full-scale application of anaerobic digestion process with partial ozonation of digested sludge Wat. Sci. Tech. 52(1-2), 245-252.

http://www2.lwr.kth.se/personal/personer/levlin_erik/eindex.htm

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