Thermal Hydrolysis of Sewage Sludge and the Recovery of Phosphate and Ammonium from Centrate Water

van Dijk, l., Sustec Consulting & Contracting bv, Netherlands


Abstract Recovery of phosphorous and ammonium from concentrated wastewater streams is getting more and more important nowadays. Phosphorous is a raw material, which is disappearing in ashes of sewage sludge, etc. Several studies predict a scarcity of phosphorous in the near future. Ammonium is produced out of nitrogen gas by fertilizer companies using natural gas (Haber Bosch process). Finally this ammonium ends up in WWTP where it is oxidized to nitrogen gas, using energy (aeration) again.

In anaerobic digestion processes ammonium and phosphorous are produced as soluble compounds, reaching high concentrations in the centrate water. When prior to the digestion of sewage sludge thermal hydrolysis is applied, very high concentrations of ammonium and phosphate will be reached in the centrate water.

Sustec applies its own continuous thermal hydrolysis system, TurboTec® process, before digestion of sewage sludge. In these cases the centrate water contains 2000 – 3000 mg NH4-N/l and up to 500 mg P/l. For the treatment of this centrate water Sustec investigated the combination of struvite formation for phosphate recovery in combination with the recovery of ammonium. This so called NutriTec® concept is a new step for recovery of ammonium, resulting in a compact, sustainable and energy efficient method compared to classical methods like biological processes as anammox.

Keywords: thermal hydrolysis, digestion, dewatering, centrate water treatment, ammonia recovery, phosphate recovery, struvite

Introduction: Treatment of domestic wastewater is an energy consuming process. On a wwtp the net energy consumption is approximately 20 – 30 kWh per year per person equivalent treated wastewater, although wastewater contains approximately 130 kWh of potential chemical energy. In the Netherlands this potential energy content is equivalent with 2 billion kWh per year (about 2% of the Dutch electricity production). The nitrogen in the wastewater is mainly oxidized to N2 gas. With this process another 37 kWh per person per year of potential energy is destroyed (to produce 1 kg of ammonia for fertilizers from N2 about 2 Nm3 of natural gas is needed). A lot of phosphorous ends up in the sludge. Due to higher concentration of nutrients in side streams ammonia as well as phosphorous can be recovered from these side streams.

In the Netherlands the wastewater treatment plants should become more energy efficient by 2% every year up to 30% in the next 12 years. At this moment many initiatives started to make the wwtp more energy efficient and even energy producing. By adding step by step several processes to the current wwtp, the plant can be made more energy efficient and valuable components like nutrients can be recovered (see figure 1).

One of the first steps, which can be introduced in this sustainable wwtp concept, is thermal hydrolysis of the surplus sludge and recovery of ammonium en phosphorous from the centrate water. Sustec has erected a continuous thermal hydrolysis of surplus sludge by the TurboTec® process at the WWTP Venlo and is constructing a TurboTec process in combination with nutrients recovery at the WWTP Apeldoorn (see step 2 and 5 in figure 1).



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