# Lessons learned from phosphorus chemical precipitation in small wastewater treatment plants

Sophie Besnault, Stéphanie Prost-Boucle, Luc Seranne, Alan Le Bouder, Jean-Marc Choubert, Sylvie Gillot, Pascal Molle

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Sophie Besnault, Stéphanie Prost-Boucle, Luc Seranne, Alan Le Bouder, Jean-Marc Choubert, et al.. Lessons learned from phosphorus chemical precipitation in small wastewater treatment plants. 6th International Conference on eco-Technologies for Wastewater Treatment, IWA, Jun 2023, Girona, Spain, Spain. [hal-04781499](https://hal.science/hal-04781499v1)

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Submitted on 13 Nov 2024

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# Lessons learned from phosphorus chemical precipitation in small wastewater treatment plants

Sophie Besnault*, Stéphanie Prost-Boucle*, Luc Seranne*, Alan Le Bouder**, Jean-Marc Choubert*, Sylvie Gillot* and Pascal Molle*

* INRAE UR REVERSAAL, 5 rue de la Doua, CS 20244, F-69625 Villeurbanne, France

(E-mail: sophie.besnault@inrae.fr)

** Conseil Général of Gironde department, SATESE 33, Esplanade Charles de Gaulle, 33074 Bordeaux Cedex, France

**Abstract**

Feedback from 46 small to medium wastewater treatment plants equipped with chemical precipitation with iron chloride for phosphorus removal was gathered. Chemical precipitation with iron chloride is efficient to reach the phosphorus levels required for all the processes studied, except when the requirements are too low (< 1 mg TP/L). But an injection of iron chloride in a small WWTP requires a more complex and more frequent follow-up and significantly increases the operation costs.

**Keywords (maximum 6 in alphabetical order)**

Iron chloride; phosphorus removal; rotating biological contactors; vertical flow treatment wetlands; trickling filters

## INTRODUCTION

The accumulation of phosphorus discharged in the environment by wastewater treatment plants (WWTPs) is one of the main causes of eutrophication. As a consequence, in France, an increasing number of small WWTPs are required to treat phosphorus, sometimes to reach very low levels, under 1 mg Total Phosphorus (TP)/L. Conventional processes installed to treat wastewater in small plants such as vertical flow treatment wetlands (VFTWs), rotating biological contactors (RBC) or trickling filters are not designed to remove phosphorus (Cramer et al., 2016). Chemical precipitation is then often implemented in order to reach the required levels of phosphorus in treated wastewater (Di Capua et al., 2022). This process is very frequent in large WWTPs, in particular in activated sludge systems (Strom, 2007). However, there is a lack of feedback on the use of chemical precipitation for phosphorus removal associated with technologies for small WWTPs (Bunce et al., 2018). Implementing chemical precipitation on smaller treatment plants presents new challenges as they cannot benefit from economies-of-scale, rigorous monitoring and in-house operating expertise as the larger WWTPs. For this reason, the French national working group Epnac (www.epnac.fr, gathering all the public organisms working on wastewater treatment in France) started a study in 2020 on phosphorus chemical precipitation implemented in different treatment technologies in small WWTPs. The objectives were to evaluate the performance of phosphorus treatment and to identify potential bottlenecks.

## MATERIALS AND METHODS

The first step of the study was a national survey to collect data (water treatment regulatory monitoring, information on the phosphorus removal process selected at the plant, feedback from the operation of this process...) from the members of the Epnac group. Specific sites were then selected to proceed to extended monitoring in order to evaluate the performance of the process for phosphorus removal using a common experimental methodology.

Data from 367 regulatory monitoring (24-hour-flow composite samples) in 46 WWTPs treating phosphorus with a capacity of 260 to 8 000 Population Equivalents were gathered. The processes implemented at the studied plants were activated sludge systems (9 treatment plants), rotating biological contactors (16), vertical flow treatment wetlands (4), VFTWs with forced aeration (5) and trickling filters combined with VFTWs (12). All these WWTPs are equipped with a chemical
precipitation unit using iron chloride ($FeCl_3$) to reach the low levels of phosphorus required at the outlet.

31 specific 24-hour-flow composite samples monitoring were produced for this study with additional parameters in regard to the classical parameters followed during the mandatory self-monitoring of the WWTP. The extended monitoring consisted in flow proportional sampling at each treatment steps in order to analyse global parameters as well as additional relevant parameters regarding phosphorus chemistry (iron, bicarbonate ions, volatile fatty acids...).

## RESULTS AND DISCUSSION

### Global performance of chemical precipitation for phosphorus removal in small WWTPs

Chemical precipitation with iron chloride is efficient to reach the phosphorus levels required except when they are too low (lower than 1 mg TP/L) as shown in Table 1. This confirms that levels of total phosphorus under 1 mg/L are hard to reach with chemical precipitation alone and require additional techniques such as filtration (Bunce et al., 2018).

<table><caption>**Table 1.** Percentage of 24-hour-flow composite samples meeting the phosphorus levels required at the outlet of the WWTPs according to the TP level required</caption><thead><tr><td>Phosphorus level required (mg TP/L)</td><td>Number of 24-hour-flow composite samples</td><td>Percentage of samples under the required phosphorus level</td></tr></thead><tbody><tr><td>&gt;=0.7 and &lt;=1</td><td>7</td><td>43%</td></tr><tr><td>&gt;1 and &lt;=1.5</td><td>34</td><td>85%</td></tr><tr><td>&gt;1.5 and &lt;=2</td><td>235</td><td>88%</td></tr><tr><td>&gt;2 and &lt;=4</td><td>62</td><td>69%</td></tr><tr><td>&gt;4</td><td>26</td><td>77%</td></tr></tbody></table>

As seen in Table 2, the activated sludge system seems to be the most reliable process as 97% of the samples are below the required phosphorus level. Other processes such as trickling filters + VFTWs or VFTWs with forced aeration allow a good compliance with the total phosphorus discharge requirements (compliance over 75%).

<table><caption>**Table 2.** Percentage of 24-hour-flow composite samples meeting the phosphorus levels required at the outlet of the WWTPs according to the process</caption><thead><tr><td>Process</td><td>Number of 24-hour-flow composite samples</td><td>Percentage of samples under the required phosphorus level</td></tr></thead><tbody><tr><td>Activated sludge</td><td>117</td><td>97%</td></tr><tr><td>Rotating biological contactors</td><td>135</td><td>61%</td></tr><tr><td>Trickling filters + VFTWs</td><td>76</td><td>75%</td></tr><tr><td>VFTWs</td><td>30</td><td>70%</td></tr><tr><td>VFTWs with forced aeration</td><td>10</td><td>80%</td></tr></tbody></table>

As shown in Figure 1, TP removals above 67% were obtained for all the processes. Concentrations of phosphorus in treated water were the lowest with activated sludge. Low concentrations could also be obtained with VFTWs with forced aeration. Lower levels of total suspended solids (TSS)
are associated with a better removal of phosphorus as around 2-3% of organic solids is phosphorus (Strom, 2007).

<figure><img src="image_2.png" /><figcaption>**Figure 1.** Comparison of processes for phosphorus removal.</figcaption></figure>

## Lessons learned

As chemical precipitation and sludge management can be implemented differently, even in a particular biological treatment technology, it is interesting to analyse more deeply the effect of the choice of the precipitation step implementation on efficiencies. A comparison of the different configurations for each process allowed to draw conclusions. For rotating biological contactors for example, 3 of the 16 WWTPs had an injection of iron chloride inside the biological rotating disc tank as shown in Figure 2. A similar removal of phosphorus (77%) was measured for these plants. In comparison with WWTPs where iron chloride was injected before or after the rotating discs tanks, a similar performance was obtained while lower iron chloride dosage was used (226 mg/l of commercial iron chloride for the injection inside the rotating discs, 98 mg/l for the injection before the rotating discs). The rotation of the discs is very slow and does not allow a good mixing of iron chloride, necessary to precipitate P. Iron chloride, as in activated sludge systems, should be injected in the most turbulent part of the tanks.

<figure><img src="image_3.png" /><figcaption>**Figure 2.** Injection of iron chloride inside the biological rotating disc tank.</figcaption></figure>

Trickling filters followed by VFTWs can have 1 stage of filters, 2 stages or a lamella clarifier as a second stage. Iron chloride is injected before or after the first stage of VFTWs, as shown in Figure 3. The WWTP with no second stage did not remove as much phosphorus (48%) and suspended solids as the WWTPs with the two other configurations (78% removal for total phosphorus). This
configuration without a second stage should be selected only when higher levels of phosphorus are required at the outlet.

<figure><img src="image_4.png" /><figcaption>**Figure 3.** Three configurations for phosphorus removal by chemical precipitation for trickling filters followed by VFTWs.</figcaption></figure>

Application of iron chloride in excess is frequent in small WWTP due to a less frequent follow-up, generating high reagent costs. Based on the data gathered in this study, the consumption of iron chloride for a theoretical 1000 PE WWTP can vary from 4.3 to 11.4 tons per year. The quantity depends on the inlet phosphorus concentration, the removal objective and the regulation of the injection. Iron chloride can represent a cost of 0.8 to 5.7 euros per year per PE excluding taxes (up to +114% on operation costs of the plant in comparison with a similar plant without P precipitation).

## Conclusions

Multiple systems exist for chemical precipitation of phosphorus in small WWTP (injection point, regulation of the injection, separation of the resulting solids residuals...) and can be efficient. It is not necessary to choose activated sludge if low levels of phosphorus are required in a small WWTP, other processes can be combined with chemical precipitation. But an injection of iron chloride in a small WWTP requires a more complex and more frequent follow-up in comparison with a classical plant without P precipitation and increases the operation costs. Phosphorus removal with iron chloride should only be implemented if the aquatic ecosystem really requires it, alternative to treatment should be considered (infiltration in the ground, reuse, evaporation...) to limit the discharge of P in superficial water.

## REFERENCES

* Bunce, J.T., Ndam, E., Ofiteru, I.D., Moore, A. and Graham, D.W. 2018 A Review of Phosphorus Removal Technologies and Their Applicability to Small-Scale Domestic Wastewater Treatment Systems. *Frontiers in Environmental Science* 6(8).

* Cramer, M., Koegst, T., Tränckner, J. 2016 Cost-efficient Phosphorus removal in rural WWTPs. 13th IWA specialized conference on small water and wastewater systems proceedings

* Di Capua, F., de Sario, S., Ferraro, A., Petrella, A., Race, M., Pirozzi, F., Fratino, U., Spasiano, D. 2022 Phosphorous removal and recovery from urban wastewater: Current practices and new directions. *Science of the Total Environment* 823

* Strom, P. F. 2007 Technologies to remove phosphorus from wastewater. *Rutgers University*, 1–8.