Sidestream Treatment: WWTP Hengelo


STP Hengelo


Hengelo, The Netherlands




NAS-ONE® volume:  3.300 m3
Influent flow: 850 m3/day
Nitrogen load: 1,500 kg N/day
Removal efficiencies: 80% TN, 90% NH4-N

Sludge processing optimisation

The Vechtstromen Water Board has converted the Hengelo STP into an sludge-to-energy plant, which produces the energy required by the sewage treatment plant and powers another 3,500 households on top. To make the treatment plant energy-positive, sludge processing was optimised. Sludge processing consists of thermal hydrolysis (THP) pre-treatment followed by mesophilic sludge digestion.

NAS-ONE® sub-stream treatment

The wastewater (reject water) of the energy plant contains such high levels of nitrogen due to the breakdown of sewage sludge that it cannot be treated according to the regular water treatment process. Colsen installed a NAS-ONE® sub-stream treatment system (UAV-GC) at Hengelo STP after it was granted the European tender for “Sidestream treatment at Hengelo STP” in December 2016.

Applied technology

The NAS-ONE® reactor at Hengelo STP treats reject water from a THP system, resulting in a greater COD and higher TSS pollutant load. Together, these factors disqualify standard anammox systems,

but thanks to advanced aeration control and extensive sludge selection methods in the NAS-ONE® reactor, this reject water can still be treated efficiently with anammox. NAS-ONE® technology can cut operating costs by 40% compared to conventional nitrogen removal methods, contributing to the overall cost and energy-efficiency of the energy plant.