Total suspended solids (TSS)

Materials in suspension are included in the indicative list of substances to be taken into account for fixing emission limit values in Annex II to the IED (2010/75/EU) [ 5, Directive 2010/75/EU 2010 ].

There are some reasons to link the analysis of TSS with other parameters. If BOD/COD/TOC removal functions poorly, TSS emissions may be affected. Conversely, high TSS values can correlate with/cause high concentrations of other parameters, namely BOD, COD/TOC, total phosphorus, total nitrogen, and metals.

It is possible to have higher TSS values in the effluent than in the influent, for example due to the growth of biomass during biological treatment or due to the precipitation of compounds during physico-chemical treatment. In most cases, it therefore does not make sense to calculate abatement efficiencies for the WWTP.

Overview of WWTP performance on TSS

Out of a total of 95 directly discharging WWTPs, TSS values in the effluent were reported for 76 WWTPs (or 80 %). Two effluent values were given as below a certain concentration or

08 118 34 113 01 86 62 85 75 07 90 40 87 54 33 11 41 64 19 91 60 53 89 111 51 70 82 100 29 74 83

Ultraf Ultraf Sed SF Filtr NI Filtr SF Sed SF NI Sed Sed Sed NI NI Sed

Flot Sed Sed NI Sed Sed Sed NI NI Sed Sed Flot Flot

Effluent values not shown in the graph:

< LOD: #69 (Bio)

< 5 mg/l: #36 (Bio)

NB: Data labels indicate the plant code (see Table 7.1 in Section 7.2, Annex II) and the type of treatment.

Bio = biological treatment; Filtr = filtration; Flot = flotation; LOD = limit of detection; NI = no information provided; NT = no final treatment; PC = physico-chemical treatment only; SF = sand filtration;

Sed = sedimentation; Ultraf = ultrafiltration.

Source: [ 246, EIPPCB 2014 ] based on data from [ 222, CWW TWG 2013 ]

Figure 2.14: Average TSS concentrations (mostly yearly averages) in the effluents of directly discharging WWTPs (all WWTPs are shown)

08 118 34 113 01 86 62 85 75 07 90 40 87 54 33 11 41 64 19 91 60 53 89 111 51 70 82 100 29 74 83 48 37 101 47 57 114 18 25

61 03 98 63 67 02 84 93 96 21 49 92 10 45 041 27 50 17 80

042 117 105 73 95 32 94 79 115 120 116 119 121

Ultraf Ultraf Sed SF Filtr NI Filtr SF Sed SF NI

Sed Sed Sed NI NI Sed Flot Sed Sed NI Sed Sed Sed NI NI Sed Sed Flot Flot SF Flot Sed Sed Flot Sed Sed Sed Flot SF Sed NI Flot SF Sed SF Sed NI Sed Flot Sed NI Sed Flot. Sed Sed NI Sed Flot NT NI NI Sed Sed NI Sed Sed NI Sed NI Sed

Effluent values not shown in the graph:

< LOD: #69 (Bio)

< 5 mg/l: #36 (Bio)

NB: Data labels indicate the plant code (see Table 7.1 in Section 7.2, Annex II) and the type of treatment.

Bio = biological treatment; Filtr = filtration; Flot = flotation; LOD = limit of detection; NI = no information provided; NT = no final treatment; PC = physico-chemical treatment only; SF = sand filtration;

Sed = sedimentation; Ultraf = ultrafiltration.

[ 246, EIPPCB 2014 ] based on data from [ 222, CWW TWG 2013 ]

08 118 34 113 01 86 62 85 75 07 90 40 87 54 33 11 41 64 19 91 60 53 89 111 51 70

61 03 98 63 67 02 84 93 96 21 49 92 10 45 041 27 50 17

042 117 105 73 95 32 94 79 115 120 116

Ultraf Ultraf Sed SF Filtr NI Filtr SF Sed SF NI Sed Sed Sed NI NI Sed Flot Sed Sed NI Sed Sed Sed NI NI Sed Sed Flot Flot SF Flot Sed Sed Flot Sed Sed Sed Flot SF Sed NI Flot SF Sed SF Sed NI Sed Flot Sed NI Sed Flot. Sed

Bio Bio PC Bio PC PC Bio Bio Bio Bio Bio PC Bio Bio PC PC PC Bio Bio Bio Bio PC Bio PC Bio PC Bio Bio Bio Bio PC Bio Bio PC Bio Bio PC Bio Bio PC Bio Bio Bio Bio PC PC Bio PC PC Bio Bio PC Bio Bio Bio

Effluent values not shown in the graph:

< LOD: #69 (Bio)

< 5 mg/l: #36 (Bio)

NB: Data labels indicate the plant code (see Table 7.1 in Section 7.2, Annex II) and the type of treatment.

Bio = biological treatment; Filtr = filtration; Flot = flotation; LOD = limit of detection; NI = no information provided; NT = no final treatment; PC = physico-chemical treatment only; SF = sand filtration;

Sed = sedimentation; Ultraf = ultrafiltration.

Source: [ 246, EIPPCB 2014 ] based on data from [ 222, CWW TWG 2013 ]

Figure 2.16: Average TSS concentrations (mostly yearly averages) in the effluents of directly discharging WWTPs (only WWTPs with effluent values ≤ 50 mg/l are shown)

WWTPs showing either relatively high BOD5 and/or COD/TOC values in the effluent (e.g. #18,

#25, #28, #57, #108, #110) tend to have higher TSS emissions.

Techniques reported to reduce TSS emissions

The techniques used for final solids removal are given in Section 2.2, as well as in Figure 2.14, Figure 2.15, and Figure 2.16. Sedimentation is most widely used, followed by flotation.

Filtration and flotation are often preceded by a sedimentation step.

Fluctuations of emissions around the average (in concentration)

Maximum TSS values reported for 58 WWTPs vary around the average by a factor of 1.3–23, but more generally by a factor of 1.6–9.2 (10th to 90th percentile). TSS fluctuations are significantly higher than COD and TOC fluctuations. Seasonal variations (e.g. rain season) and the treatment of storm water (sometimes from a nearby city) may cause higher variability in the TSS effluent concentration.

Limits of detection (LOD) and quantification (LOQ)

In Flanders (Belgium), TSS is considered not quantifiable below 2 mg/l. In Spain, the LOQ for TSS is 2 mg/l. Two international standards, EN 872 (2005) and ISO 11923 (1997), describe a method for the determination of suspended solids in raw waters and waste waters by filtration though glass fibre filters. The lower limit of the determination is in both cases approximately 2 mg/l.

Parameters that affect performance

The performance of the techniques used for final solids removal is influenced by the characteristics (e.g. settleability) and concentration of the suspended solids. Biological WWTPs with high influent loads of BOD (e.g. some OFC plants) generally generate more biomass

Relationship between performance and techniques used as reported in the questionnaires

The performance of the WWTPs with respect to the removal of solids depends on the techniques used, but also on the characteristics of the waste waters and the operating conditions of the installations (e.g. residence time in the final clarifier in the case of sedimentation).

Average TSS values below 25 mg/l are achieved by WWTPs using sand filtration (e.g. #34:

5 mg/l, #01: 6 mg/l, #61: 6 mg/l, #32: 15 mg/l, #115: 21 mg/l, #45: 22 mg/l, and #120: 23 mg/l).

Installations using ultrafiltration as the main solids removal step (often as part of a membrane bioreactor) show very low TSS values in the effluent (i.e. #36: < limit of detection, #08: 1 mg/l,

#118: 2 mg/l, and #69: < 5 mg/l). In the case of the installations #41 and #70, only part of the effluent is treated by the membranes, and therefore TSS emissions are higher (17 mg/l and 48 mg/l, respectively).

Directly discharging WWTPs using flotation tend to show higher TSS values in the effluent (15–120 mg/l), although nine out of the eleven WWTPs reporting the use of flotation achieve average TSS values below 35 mg/l (i.e. #10, #11, #21, #33, #35, #50, #60, #64, #75) and five of them even below 20 mg/l (i.e. #11, #21, #33, #64, and #75). The largest spread of TSS effluent values is found when sedimentation as the main solids removal step is used (5–2 900 mg/l).

Nevertheless, among the 34 directly discharging WWTPs using sedimentation and reporting TSS emission values, 17 WWTPs achieve average TSS values below 20 mg/l, and 23 WWTPs achieve average TSS values below 35 mg/l.