Calculation of Chemical
Needs in Combined
Phosphorus Removal and
Recovery at Henriksdal
WWTP, Sweden

 

K. Stark, B. Hultman, E. Levlin, M. Lwn and A. Mossakowska*

 

Dep. Land and Water Resources Engineering, Royal Institute of Technology

*Stockholm Water Co

 

 

INTRODUCTION

 

Problems with sludge disposal and expected demands to recover phosphorus have led to reconsideration of the operational scheme at Henriksdal wastewater treatment plant in Sweden. In a study simple models were developed to calculate the chemical demand for different operational schemes including use of different chemicals in chemical precipitation and different degrees of biological phosphorus removal. The calculation model is in agreement with reported studies on the phosphorus recovery systems Cambi/Krepro and BioCon. In this poster is presented the results of chemical demand of different technology systems for phosphorus recovery at present operational conditions at Henriksdal wastewater treatment plant both by calculation and compared with performed experiments.


 


Figure 1 Connection between used amount acid and base at the dissolving step and recovery step by use of the systems KREPRO, Cambi/KREPRO and BioCon (Hultman et al, 2001)

 

 

Calculation Model

 

A calculation model has been developed to predict chemical demand of acids and bases at transferation of particle bound phosphorus to a liquid phase followed by product recovery from the liquid phase (Hultman et al, 2001 and Stark et al, 2001). It was applied to Henriksdal WWTP by different operation design.


The model consists of three parts:

(1) Chemicals needed for phosphorus removal either used for chemical precipitation (use of aluminium or iron salts, lime and chemicals for pH-adjustment) or for enhanced biological phosphorus removal (addition of for instance acetic acid).
(2)Chemicals needed for transferring sludge-bound phosphorus into a liquid phase (for instance use of acids or bases).
(3)Chemicals needed for transferring soluble phosphorus into a product.

 


 


Figure 2 Effect of weight relationship Fe/P of total chemical demands of acid and base
for the systems KREPRO, Cambi/KREPRO and BioCon (Hultman et al, 2001)

 

 

Results from Calculation

 

The calculation model on chemical demand for phosphorus recovery shows that the demand for dissolving inorganic components by use of acids is due to influent inorganic constituents that will form part of the sludge and the inorganic sludge formed due to addition of precipitation agents. The amount of chemicals needed for dissolution of inorganic chemicals is about the same as the chemical needs, calculated in kg/ton dry solids, to obtain phosphorus as a product (figure 1). With present mode of operation at Henriksdal WWTP about 800 kg chemicals (acids and bases) are needed for phosphorus recovery independent of chosen technology for phosphorus recovery (figure 2). The chemical demand could be reduced to about 400 kg chemicals if the operational mode was changed to biological phosphorus removal followed by minor dosage of ferrous sulphate to the filters. As mentioned, BioCon and KREPRO use approximately the same amount of chemicals. In both cases will the acid be used to 1) solve the inorganic material in the sludge which is entering together in the sewage 2) solve the iron from precipitation agents. Thereafter will the chemical (approximately the same amount) to precipitate the different inorganic component incl. Iron phosphate (KREPRO) or generating from ion exchanger BioCon.


From figure 2, the chemical demand Kv can be expressed with terms in weight units (kg chemicals/ton DS) according to:

Kv = 200 + 160 * Fev/Pv

 

where

Fev = iron content in sludge, kg Fe/ton DS

 

Pv = phosphorus content in sludge, kg P/ton DS


The line crossing with the y-axle in figure 2 means the amount of chemicals to release of inorganic matter not depending on the iron content. It might be inorganic components in the wastewater entering the WWTP (calcium- and magnesium carbonates, zeolites etc).

 


 


Figure 3 Amount of acid used to release the phosphorus in the digested sludge

 

 

 

ACKNOWLEDGEMENTS

The study has been supported by MISTRA, as a part of a research program called Urban Water, and
by Stockholm Water Co. Ttravel schoolarship from Knut and Alice Wallenberg Foundation.

 

 

REFERENCES


Hultman, B., Levlin, E., Lwn, M., Mossakowska, A. and Stark, K. (2001).
Recovery of phosphorus and other products from sludge and ash, Stockholm Water Co, R no 6, March 2001. (in Swedish)
Stark K., Hultman B., Mossakowska A. and Levlin E. (2001a). Chemical needs in phosphorus recovery from sewage sludge. Vatten 57, 3, pp. 207-215. (in Swedish)
Stark K. (2001b). Phosphorus release from sewage sludge by use of acids and bases, Proceedings of a Polish-Swedish seminar, Nowy Targ - Zakopane October 24-26, 2001 Wastewater sludge and solid waste management, Report No 9 ISBN 91-7283-190-1. pp. 19-30.

 

 

Comparing with Experiments


Introductory experiments with sludge fractionation using acid and base have been carried out at department Land and Water Resources Engineering, Royal Institute of Technology, (KTH) in Stockholm, Sweden (Stark, 2001). In one of the experiment hydrochloric acid (HCl) was added to digested sludge at different pH-values and mixed for 2h in room temperature. Total phosphorus amount released, the chemical demand and the suspended solids were analysed. In figure 3 is shown the result of how much phosphorus is released at different amount of acid. To release 80% of the phosphorus in the digested sludge about 250 kg HCl/ton SS is needed which is in good agreement with the expected values (Hultman et al, 2001 and Stark et al, 2001).

 

 

CONCLUSION


To receive a high phosphorus recovery (over 75%) addition of chemicals are needed to first solve the inorganic sludge components (including phosphate) and then chemicals are needed to create a product of phosphate. The heavy metals are separated in the process, which receives a clean product.
With present mode of operation at Henriksdal WWTP about 800 kg chemicals (acids and bases) are needed for phosphorus recovery independent of chosen technology for phosphorus recovery.


Main conclusions from the calculation work are:

 

  • The operation in Henriksdal wastewater treatment plant should minimise chemical additions and try to improve the degree of enhanced biological phosphorus removal
  • External addition of organic material (such as acetic acid) is very expensive and different methods to internally produce organic material for improvement of enhanced biological phosphorus removal should be used
  • If iron is used as a precipitation agent dissolution of iron in phosphorus recovery in ferrous form is advantageous compared with dissolution in ferric form
  • Special advantages may be obtained by use of aluminium salts or lime in chemical precipitation compared with the use of iron salts (dissolution at high pH-values or use of thermal technologies)

Preliminary experiments works are in good agreement with the calculations.