HUBER flocculation filtration for the elimination of phosphorus

HUBER Sandfilter CONTIFLOW® CFSF 51

The federal cabinet decided in 2017 to revise the German Waste Sewage Sludge Ordinance. The revised ordinance includes the restricted spreading of sewage sludge on agricultural land. Sewage sludge contains the finite resource phosphorus that has a reinforcing effect on eutrophication Eutrophication is the overloading of waters with the nutrients nitrogen and phosphorus. More stringent effluent standards for sewage treatment plants reduce such overfertilisation of surface waters and, at the same time, increase the recoverable concentration contained in the sewage sludge. The economical and considerate exploitation of the finite nutrient phosphorus is ensured. The fact that these two provisions have been made underlines the enormous importance of this resource for humans and animals.

Phosphorus is an irreplaceable substance for the fertilizer industry. The resource is mainly bound within rocks and neither Germany nor the rest of Europe are possessing any reserves. The German industry therefore depends on the import of phosphorus. In 2014, The European Union put phosphorus on the list of “critical raw materials”. A possibility to gain some independence from imports is to utilize the available phosphorus compounds as efficiently as possible. Due to its high concentration in wastewater, the recovery of the phosphorus contained in sewage sludge and the reduction standards for the effluent from sewage treatment plants are increasingly coming into focus.

Every German citizen produces approximately 1.8 g / PT phosphorus a day. The resulting inlet concentrations of total phosphorus [Ptotal] on sewage treatment plants range between 5.0 and 15.0 mg/l. The phosphorus consists of two main fractions, the dissolved and the particulate fraction.

HUBER Sandfilter Contiflow® CFSF 51 – function diagram

One distinguishes basically between the two variants of biological and physical-chemical phosphorus elimination. The physical-chemical processes are described in the following. Physical processes (sedimentation in preliminary and secondary clarifiers or filtration, e.g. sand filtration) as part of the treatment process on a sewage treatment enable the removal of the particulate phosphorus. The dissolved phosphorus can virtually not be reduced without adding chemicals. On its way to the sewage treatment plant, this fraction is nearly completely converted into anorganic orthophosphate. If this fraction needs to be retained, it must be transferred into a particulate state. This chemical process is called a precipitation reaction. The concentration of filterable solids (=AFS) in the wastewater flow increases. Precipitation reactions can be carried out at very different places within the sewage  treatment plant (preliminary precipitation, simultaneous precipitation, postprecipitation).

The dose and dosing point depend on the required monitoring value and the type of chemical used (mostly aluminium or iron salts). For a required P-effluent value of <= 1.0 mg/l for example, simultaneous precipitation in or upstream of the biological treatment stage is used very frequently. Stoichiometric precipitant overdoses are defined in dependence of the reduced concentration of phosphorus within the wastewater. This ratio is referred to as beta factor. The higher the spread between the inlet concentration and the required effluent concentration, the more precipitant must be added.

To obtain P-effluent values of <= 0.1 mg/l, it is indispensable in many case to install a fourth treatment stage specifically for the elimination of phosphorus. An efficient process technology for the fourth treatment stage is flocculation filtration with the HUBER Sandfilter CONTIFLOW®. Flocculation filtration means the addition of precipitant doses into the inflow to a sandfilter. To operate such a phosphorus elimination system economically, the phosphorus concentration is reduced in advance to < 1.0 mg/l in an upstream precipitation stage (for example simultaneous precipitation).

Due to the fast reaction velocity of the precipitant with the orthophosphate, the filter bed of the HUBER Sandfilter CONTIFLOW® can directly be used as reaction volume for the conversion and retention of the particulate phosphorus. The required effluent values for phosphorus and AFS are reliably met in this way.

Large-scale filtration plant - 6x HUBER Sandfilter Contiflow® CFSF 51 units

Functional description

The HUBER Sandfilter CONTIFLOW® is a deep-bed type upflow filter with continuous filtration and discontinuous filter bed cleaning (1.0 – 2.0 mm sand grain size) that meets even the highest effluent quality requirements. The filter is highly efficient as no shutdowns are necessary for the sand washing process.

Depending on the specific local conditions on site, the system is installed in a stainless steel tank or in a concrete tank. Its modular design ensures the optimal treatment of any throughput. Depending on the required effluent quality or inlet concentration, the system uses filtration velocities of up to 14.0 m/h.

The feed is introduced into the HUBER Sandfilter CONTIFLOW® through a feed line which leads to a series of feed radials at the bottom of the filter. As the influent flows from the bottom upward through the sand bed (mostly 2.0 m sand bed height), the solids contained within the influent are retained in the filter sand (solids including particulate phosphorus). The filtrate exits over a weir at the top of the filter.

As the loading of the filter bed with solids increases, the primary pressure required for the flow through the filter rises (Δhmax < 1200 mm). When a defined pressure switch point is reached, the solids (including the particulate phosphorus) are delivered upwards by an airlift into the washer unit. Inside the washer, the solids are separated from the filter sand due to the different settling velocities.

They are washed out along with a small portion of the filtrate flow, the so-called wash water. The clean sand falls down through to the bed, generating an internal sand circulation in the HUBER Sandfilter CONTIFLOW®.