19.2.1 Description of an STRB

Sludge Treatment in Reed Bed Systems are vertical flow constructed systems for the dewatering and stabilization of sludge. Sludge treatment reed beds comprise a series of basins, which are vegetated typically with common reeds (Phragmites australis) in a filter [10]. The sludge residue remains on the surface of the filter in the basins and is mineralized through the natural biophysical interaction of plants, microbes and air, whilst water is removed from the sludge by both evapotranspiration and drainage through the sludge residue and filter (Figure 19.1) [10].

Vegetation and ventilation with air in STRB systems create favorable conditions for the transformation of degradable organic matter to a more stable humic form [11, 12]. As the organic matter mineralizes, the overall sludge volume reduces and the accumulating sludge residue becomes continually part of the filter in which the reeds grow. The height of the mineralized sludge residue layer increases with time with an accumulation rate of approximately 10–15 cm/yr, depending on the sludge quality. After typically 8 to 12 (up to 20) years of operation, the dewatered and mineralized sludge residue is excavated from the STRB basins and recycled as fertilizer or as soil conditioner.

Currently, STRBs are used primarily for treatment of sewage sludge from municipal wastewater treatment plants (WWTP), but the technology is also used for the treatment of water works (WW) sludge and sludge originating from the agro-industrial production. However, some types of sludge may not be suitable for dewatering in a STRB, e.g., if the sludge has a high concentration of fat and oil. Experience has shown that STRBs are capable of treating most types of sludge with dry solid concentrations between 0.1–5%.

19.2.2 Description of STRB Test-System

The dewatering and treatment efficiency of sludge in STRBs depends on the sludge quality, and the design and operation of the STRB. Before planning and establishing of new full-scale STRB plants, it is strongly recommended to test the sludge's dewatering properties in a test system with similar filter and setup as the intended full-scale system [7]. A test of the dewater ability of the sludge is especially important, if it is not ‘normal’ sludge from households, but industrial sludge with, for example, a high content of fat, oil, organic material or heavy metals.

In Denmark, Germany, Sweden, France and other countries in Europe the design and dimensioning of STRBs has been extremely variable during the last 20 years, even if they were treating the same sludge type. The number of basins in the different systems varied between 1–24 basins, basin areas between less than 100 m² to over 3,000 m² and the area load varied between 30 to over 100 kg ds/m²/yr [7]. Consequently, the outcome of the sludge treatment has also varied. If the STRB has been properly designed and operated, high dry solid contents in the sludge residue has been obtained, while the opposite has been true at poorly designed and operated STRBs [9]. Some sludge types are difficult to dewater by both STRB and mechanical treatment technology.

A test system consists typically of 3–6 pilot-scale basins of 1–2 (up to 100) m² with filter, drain layer, drains, growth layer and reeds (Phragmites). During the trial period of typically 4–12 months (sometimes it is necessary with 2 years), each basin is loaded several times, followed by a resting period, where no sludge is added. The length of the loading and rest periods depends on the sludge characteristics, climate (cold climates need longer resting periods), and the age of the system and specific basin, the dry solid content and the thickness of the sludge residue.

The main purpose of a test (phase 1) is to test whether the sludge would be suitable for further treatment in a STRB or not [7]. This will be answered by gathering information about:

• Sludge quality and characteristics.
• Dewatering efficiency of the sludge (L/sec/m²).
• The sludge residue behavior (dry/crack up) in a trial bed.
• The growth of reeds. Is it possible to get the vegetation to grow in the sludge?

Later on in the test period in phase 2 the loading onto the reed beds will be much more intensive. Loading rates will vary in the test period so that the differences in load/rest ratio and area load can be more clearly defined. Different numbers of loading days and resting days will be tested. The main purpose in phase 2 is to test (ascertain the criteria) for the dimensioning and operation of a full-scale system [7]. For example:

• What loading (kg dry solid/m²/yr) can the trial system treat?
• The number of basins that is necessary in order to get the desired resting period between loadings.
• How many days can we load in a loading period?
• How many days rest are necessary for drainage and drying (min and max)?
• Load and rest program in relation to sludge quality.
• Determine sludge residue and filtrate water quality.
• Summer and winter operation.
• Determine the sludge quality.
• Determine sludge residue and filtrate water quality.
• Finally, to give recommendations for up-stream changes on the works in order to reduce unwanted parameters in the feed sludge (e.g., heavy metal, fat and oil etc.)

19.3.1 Organic Material in Sludge

Insufficient dewatering of sludge in an STRB and in mechanical dewater equipment also, is often due to the quality of the sludge and sludge residual. The ratio between organic and inorganic material is an important parameter in the evaluation of sludge dewatering properties [9]. The organic content is determined typically by measuring the ‘Loss on Ignition’ (LOI).

The water retention capacity of sludge with a high proportion of organic material is many times larger than more inorganic sludge, as the content of organic material has major influence on the content of free water. The higher the organic material, the lower the content of free water and the higher the content of capillary water [20], which is more difficult to dewater [21].

When comparing the feed sludge's content of organic material (expressed as LOI) with the corresponding dry solid content in the sludge residue for a large number of STRB basins, there is a clear tendency that higher contents of organic material in the feed sludge results in lower dry solid contents in the sludge residue in the STRB (Figure 19.2).

19.3.2 Fats and Oil in Sludge

An important visual indication of a proper dewatering of the sludge is that the upper layer continually becomes cracked and broken during the resting phase. Sludge containing a high proportion of fats and oil often has a tendency not to crack and break open during the resting phase, reducing thereby the aeration from the surface considerably. If this cracking does not occur, it is due to ineffective dewatering of the sludge [21].

A high content of fats and oil in the feed sludge is especially important to address and reduce, since this, like organic material, has a pronounced effect on the dry solid content in the sludge residue in the STRB (Figure 19.3). Experience has shown that a fat content in the feed sludge above 5,000 mg/kg ds considerably reduces the dewatering and results in pronounced anaerobic conditions in the sludge residue, which becomes black and smelly [21].

19.3.3 Heavy Metals in Sludge

Depending on the industry producing the wastewater, high contents of heavy metals may be present in sludge. The concentration of heavy metals may fluctuate over time, due to changes in the production at the industry producing the wastewater. Sludge, which normally has low levels of heavy metals below legal limits, may, e.g., if new industry is connected to the WWTP, rise to levels inhibitory for reeds or reach levels in the sludge residue, which exceed legal limits for agricultural use (Table 19.1).

Experience from Denmark has shown that the level of heavy metals in the sludge residue after 10 to 20 years of treatment in STRBs is in general low – well below the Danish and EU legal limit values [22]. Heavy metals are mainly bound to particles in the sludge residue and filter matrix, and the translocation of heavy metals out of the systems with the drainage is, therefore, limited [11, 15, 22, 23]. A study by Stefanakis and Tsihrintzis from 2012 [24] about heavy metals in an STRB showed that the major sink in the mass balance for heavy metals was the gravel drainage layer, while accumulation in the sludge layer and plant uptake was low. Less than 16% of the heavy metals left the bed through drainage. In Denmark, there have been cases with reeds dying out due to nickel pollution in the sludge residue at a WWTP with connected heavy industry.

19.3.4 Nutrients in Sludge

The sludge residue from an STRB is valuated as a fertilizer, since the phosphorus and nitrogen concentration is typically high due to anthropogenic origin of the sludge [25]. Nitrogen typically decreases during the treatment period and with the depth of the sludge residue in the basins, due to the microbial mediated processes of nitrification and denitrification [11]. In addition, some nitrogen leaves the system with the drainage water (filtrate water) mainly in the form of nitrate and is returned to the WWTP.

Total phosphorous concentration has been shown to increase for the whole sludge residue in an STRB compared to the feed sludge [15], due to the mineralization of organic material in the sludge residue. Part of the phosphorous interacts with iron and other constituents in the sludge and is, thus, bound in the sludge residue matrix.

19.3.5 Hazardous Organic Compounds in Sludge

Various hazardous organic compounds may be present in sludge, such as polyaromatic hydrocarbons (PAHs), di-2-ethylhexyl-phthalates (DEHPs), nonylphenol/nonylphenol ethoxylates (NPEs), linear alkyl benzene sulfonates (LASs), originating, respectively, from coal and tars, lubricating oil additives and detergents. Studies have shown that hazardous organic compounds are mineralized during the treatment in STRB [9, 26]. Unlike traditional mechanical dewatering, such as centrifuges, the long treatment time of the sludge residue in STRBs (typically 8–10 years or more) provides sufficient time for microbiological and non-biotic mineralization processes to effectively degrade and reduce most of the hazardous organic compounds. Even within a shorter timespan of 3–6 months, significant reductions have been shown [8, 22].

In a study of the mineralization of LAS and NPE in digested sludge treated in an STRB, a degradation of 98% of LAS and 93% of NPE was obtained under aerobic conditions ([9]; Danish Environmental Agency: Report no. 22, year 2000). The study showed that oxygen was the limiting factor in the degradation of the organic contaminants. Oxygen influx into the sludge improved the mineralization of LAS and NPE considerably, while mineralization under anaerobic conditions was very limited. In the same study, reductions of approximately 60 and 32% in an STRB were obtained for DEHP and PAH, respectively. The organic contaminants were not only mineralized in the upper layer of sludge residue, but in the whole depth. In storage experiments with anaerobically digested sludge (representing mechanical treatment) LAS, NPE, DEHP and PAH were only partly degraded in the top layer (0–20 cm) and below 20 cm, no degradation occurred.

19.4.1 Case 1: Treatment of Industrial Sewage Sludge with High Contents of Fat

The industrial sewage sludge originates from WWTP treatment of wastewater with a large input from industry. Table 19.2 presents Danish STRBs systems treating sludge originating from treatment of wastewater with major input from industry: Tinglev STRB (abattoir), Kolding STRB (abattoir), Skagen test STRB (fishing industry), Skive STRB (abattoir) all Danish STRBs and in Kristianstad (Sweden) test STRB receiving sewage water from the food industry (dairies, abattoir representing (20–25%), chicken and others). These systems all receiving sludge with a high proportion of organic material. The sludge quality was characterized by a high loss of ignition (between 65 and 76% of dry solids), and with high contents of fat (15,000–30,000 mg/kg ds) and oil (2,300–7,000 mg/kg ds). Results of dewatering from these STRB systems are presented in Table 19.2 [21].

The high contents of fat and oil resulted in a dewatering profile with a maximum level approximately five to ten times lower (only 0.001–0.004 L/s/m²) than observed for normal sewage sludge (Figure 19.4) in a well-functioning STRB system (Helsinge STRB in Denmark) typically with maximum levels of 0.015–0.020 L/s/m² (Figure 19.4). It has been shown that dewatering of sludge with LOI between 50 and 65% will have a maximal drainage in the order of 0.008–0.020 L/s/m² [21].

Sludge qualities with high contents of fat and oil often result in dewatering profiles, where the maximum level of the dewatering is very low and the dewatering does not decline to zero between the loads and after the last load, but has a long “tail” continuously remaining at a certain level until the next sludge load period of the specific basin. This means that the sludge residue in that basin does not have a resting phase in between loads, resulting in an anaerobic wet sludge residue.

19.4.2 Case 2: Treatment of Industrial Sewage Sludge with High Contents of Heavy Metal (Nickel)

It is important to follow the feed sludge quality on a continuous basis, with regular sampling and analyses. If heavy metal concentrations, e.g., of nickel or another heavy metal exceeds the national legal limits (Table 19.1) for recycling sludge to agriculture in the feed sludge loaded to a STRB, the sludge residue cannot be applied on agricultural lands, but has to be deposited on a landfill or incinerated, which is much more expensive [22]. Potential inhibition of the reed growth or toxic effects is another reason for keeping a close eye on the sludge quality, here under the content of heavy metals in the feed sludge.

The Danish STRB (Stenlille) is an example of an STRB which has received sludge with high nickel levels (Figure 19.5). The vegetation (Phragmites) in the basins was visibly not thriving with thin yellow new shoots. The Danish national legal limits for sludge applied to agricultural lands is 30 mg/kg ds (Table 19.1).

As can be seen in Figure 19.6, nickel in the feed sludge was on most occasions several times higher than the national limit (red line in Figure 19.6). Research has shown that nickel concentration in the reeds of more than 5 mg/kg ds may cause toxic effects on the plants [27]. The reeds at the specific STRB site had nickel concentrations of 9.7 mg/kg ds (Table 19.3).

In order to obtain good healthy vegetation and well-functioning STRB again, it is necessary to reduce the nickel concentration in the feed sludge either by the source or at the WWTP. In the specific case, the industry, which caused the nickel pollution in the wastewater, closed for a few years after 2011 and the vegetation was re-established.

19.4.3 Case 3: Treatment of Water Works Sludge

Treatment and disposal of coagulated settled water works sludge presents a great difficulty for the water industry in Europe [28]. In general, dewatering characteristics are poor and the sludge is of limited beneficial use.

Water works (WW) sludge is generated during the purification and filtration processes of low solid waters from freshwater reservoirs, lakes or rivers or from groundwater reservoirs. For the purification process, coagulants, such as aluminium sulphate or polyaluminium chloride, and Iron based coagulants, such as ferric sulphate or ferric chloride, are used for removing impurities, after which the sludge is thickened and dewatered.

The sludge quality may considerably vary from one work to another [28], but all tend to be:

• Sticky
• Difficult to handle
• Often have an unpleasant odor (dependent on their source).

In general, most dewatered sludge goes to landfill at considerable costs [28]. Alternatively, sludge is dewatered onsite or it is transported to the nearest Waste Water Treatment Plan (WWTP) for dewatering. The typical method for dewatering of water works sludge is by mechanical treatment systems. However, in the last decade full-scale STRBs and test systems have been introduced for sludge dewatering of water works sludge.

In the following sections, results are presented from sludge treatment in STRB of Water Works Sludge from Hanningfield and Whitacre Water Work, briefly described below (Table 19.4).

Hanningfield Water Works (Northumbrian Water, England)

Currently the world's largest full-scale STRB system of 4.5 hectares and 16 basins for treatment of water works sludge is from Hanningfield Water Works. Hanningfield STRB was constructed full-scale in 2012, representing a treatment capacity of 1,275 tons dry solid per year and is situated approximately 3 km from the water works (Figure 19.7; Table 19.4).

The sludge quality from Hanningfield Water Works was originally tested in an STRB test system in the period 2008–2013 [21, 29] before the full-scale system was established (Figure 19.8). A trial was set up with six basins each of 20 m² at the Hanningfield Reservoir in Essex to examine the dewatering processes of the sludge produced at the water work. The test system was built with a design comparable to a full-scale plant with reeds, ventilation, sludge input; reject water systems as well as filters and drains.

19.4.3.1 Feed Sludge and Resulting Filtrate Quality

The feed sludge quality from WW are in general characterized by a low dry solid content and a high iron or aluminium content, due to the production process at the works using ferric- or aluminium sulphate/chloride for the coagulation process.

WW sludge is characterized by a much lower dry solid content (0.1–0.2% ds), five to ten times lower than typical activated sludge from WWTP. Moreover, the level of organic material and fats and oil is generally low in WW sludge (<1000 mg/kg ds) as the sludge is generated from a production process involving mainly surface water or groundwater. Based on experience, these parameters do not cause operational problems.

Dry solid content in feed sludge from Whitacre and Hanningfield WWs were in the range 0.1–0.5% ds (Figure 19.9) and for suspended solids in the range of 100–8,000 mg/L (Table 19.5). The data show that there is a large variation in the total solids loaded onto the basins (Figure 19.9). Therefore, it is very important to know the percentage of dry solids in each load in order to calculate the load (kg ds/m²/yr) as precisely as possible. If an average “total solids” value is used to calculate the load, there is a risk that the load may be overestimated in some basins and underestimated in others.

Feed sludge from WW		Whiteacre	Hanningfield	Interval range (both WW)
Sampling STRB site:		Test STRB	Test/Full-scale
Sampling period:		Autumn 2015	2008–2013
Parameter	Unit	Average (n = 2)	Average (n > 25)	(n > 25)
Dry solids	%	0.3	0.2	0.1–0.5
Suspended solids	mg/L	2,630	1,262	100–8,000
Loss on Ignition	%	40	23	10–40
pH	–	7.3	7.4	6.8 – 8.7
Fat and oil	mg/kg ds		604	10–2400
Iron (total) as Fe	mg/kg ds	258,530	232,800	100,000–400,000
Aluminum total	mg/kg ds	432	408	100–3,000
Total nitrogen	mg/kg ds		2,300	1,000–14,000
Total phosphorous	mg/kg ds		6964	1,500–11,000
Phosphate as P	mg/kg ds	7,241		2,000–8,000
Chloride	mg/kg ds	16,301	41,568	15,000–45,000
Calcium, total	mg/kg ds	33,555	98,143	32,000–290,000

The sludge from the two WWs was characterized by relatively low contents of organic material (as determined by LOI) between 20–40% of dry solids (Table 19.5). Sewage sludge usually has organic contents of approximately 50–70% determined by LOI.

The fats and oil content for Hanningfield WW was very low (on average 600 mg/kg ds, ranging from 10–2,400 mg/kg ds). The feed sludge had a high iron content around 250,000 mg/kg ds, ranging from 100,000 up to 400,000 mg/kg ds. The pH level is within the limits to allow good growing conditions for the reeds. There is nitrogen and phosphorus in the sludge, which makes it possible to reduce or even avoid fertilizer additions. After drainage the resulting filtrate water quality shows significant reductions in dry solids (70%), suspended solids (>95%), iron content (>95%) and total phosphorous (>95%) (Table 19.6).

Filtrate water
from WW		Whiteacre	Hanningfield	Data range
Sampling site:		Test STRB	Test/Full-scale
Sampling period:		Autumn 2015	2008-2013
Parameter	Unit	Average (n = 3)	Average (n > 25)
Dry solids	%	0.06	0.05	0.001–0.06
Suspended solids	mg/L	93	0.01	0.001–0.05
pH		7.9	7.7	7.0-8.0
BOD5	mg/L	5	2.4	1–36
COD total	mg/L	44	33	3–380
Iron (total) as Fe	mg/L	29	4.5	0–120
Total phosphorous as P	mg/L	0.1	0.2	0–4.6
Total nitrogen as N	mg/L		3.3	0–10
Chloride	mg/L	52	74	50–100

19.4.3.2 Sedimentation and Capillary Suction Time

A sedimentation test of sludge is simple and quick first indication of the dewater ability of the feed sludge. Due to the usually low fat and oil, content in WW sludge, the sedimentation of suspended material proceeds rather fast, as shown in Figure 19.10 from Hanningfield STRB. The sludge settled to approximately 8 to 10 cm within 30 minutes. The filtrate water (above the settled sludge phase) is transparent without any coloring (Figure 19.10). As the sludge sedimentation characteristics may change over time due to upstream changes at the WW, it is recommended to measure the sedimentation repeatedly during a test period.

Another simple but good indication of the sludge dewater ability is the capillary suction time (CST). The CST is a measure of the dewatering properties of sludge. This method is essentially measuring how quickly the sludge wets a filter paper. A good dewatering ability corresponds to a CST range of 10–100 seconds (Figure 19.11).

The values of CST measured for WW sludge are in the higher end of what normally is observed for WWTP sewage sludge, which indicates that the sludge is somewhat more difficult to dewater. However, the higher CST values of the WW sludge did not affect the dewatering and the cracking of the sludge.

The capillary suction time (CST) of Hanningfield nd Whitacre WWs were both approximately 2–4 times (30–110 sec) the suction time in surplus activated sludge (10–40 sec). However, there are examples of digested sludge from WWTP with CST values of more than 1,000–2,000 sec. These types usually have a high content of fat and oil.

19.4.3.3 Sludge Volume Reduction and Sludge Residue Development

After a load of sludge, the dewatering phase results in the dry solid content of the sludge remaining on the filter surface as sludge residue, whereas the majority of its water content continues to flow vertically through the sludge residue. The water content is further reduced through evapotranspiration. The volume reduction is typically very high due to the low dry solid content in the sludge.

During the 5.5 months loading period in the Whitacre STRB test, there was a volume reduction of 97–98%. In the intensive loading periods with four days of loading, e.g., in basin 3 from 23^rd–27^th November, the sludge residue height increased fast, whereas the height was markedly reduced during the following resting period due to dewatering and evapotranspiration (Figure 19.12).

In the Hanningfield test STRB, the sludge volume reduction after 3–5 days of loading was of the same magnitude (>98%). In full-scale systems with much longer resting periods than in the test, the reduction is even more pronounced. The general experience from treatment of WW sludge in both test and full-scale systems is that the dry solid content (approx. 0.1–0.2%) in the feed sludge is concentrated 100–200 times to a dry solid content in the sludge residue of between >25% up to 40% ds [30]. In Hanningfield, dry solid contents of even above 50% ds in the sludge residue were obtained in some of the test basins.

The (visual) development of the sludge residue surface after loading and during the resting period is a very important indication of the dewater ability of the sludge. After a sludge load, the WW sludge typically cracks up quickly (Figures 19.13 and 19.14). A main reason for this is the low content of fat and oil and organic matter in WW sludge. For comparison, some types of sewage sludge – especially sludge originating from treatment of wastewater with a large proportion of dairy or abattoir wastewater (fat and oil) – will not crack up.

19.4.3.4 Filtrate Water Flow

The majority of the water in a sludge load continues to flow vertically through the sludge residue and filter to the drainage system. In general, the filtrate flow curve is one of the most important parameters to follow in the operation of an STRB (Figure 19.15). If the flow peak (L/sec/m²) decreases over time after each resting period or if the flow curve tail does not decline to zero in the initial part of the rest period, this indicates that the STRB basin does not work as intended.

The filtrate water curves from Whitacre and Hanningfield test systems resembled filtrate water curves normally seen in full-scale systems for normal sewage sludge with an initial top, which then declines towards zero flow within few hours (Figure 19.15). The maximum flow rate was typically between 0.015–0.03 L/sec/m². The dewatering (L/sec/m²) in the Hanningfield full-scale and test system has generally been good with no observed occurrences of surface clogging, slow dewatering or water ponding/no drainage through the sludge residue. The quality of the filtrate water strongly indicated that there is no bypass of sludge in the test basins.

Hanningfield: In some basins the maximum dewatering speed was over 0.025 L/sec/m², approximately two times higher than that seen from well-functioning STRB systems treating sludge from WWTP [30].

In summary the overall results from full-scale STRBs and test systems treating WW sludge has shown that WW sludge is treatable in an STRB system provided these systems are built and operated correctly and that the sludge quality is of the “normal” WW type.

19.5.1 Industrial Sludge

In general, higher concentrations of organic solids result in a lower dry solid percentage and more pronounced anaerobic conditions in the sludge residue. The level of fats and oil, together with the organic content and heavy metals, are main parameters to focus on and test for in industrial sludge. If the sludge quality contains high levels of fats and oil, it is important to address this upstream the STRB at the WWTP, e.g., by improving fat and grease removal and by reducing the content in the incoming wastewater to the WWTP.

Before investing in a full-scale STRB system, it is often advisable as a first step to establish STRB test systems to assess the sludge treatability in an STRB system. A thorough test in a test STRB provides valuable information about (1) if the sludge can be treated in an STRB; (2) the dimension criteria of a full-scale system, e.g., the areal load (kg ds/m²/yr) and the number of basins.

Unlike organic compounds, heavy metals are not degraded in an STRB system, but are mainly bound in the sludge residue and filter. Heavy metal concentrations may be originated in wastewater from the industry, e.g., abattoirs constitute a larger proportion, than in normal wastewater. During the sludge treatment period in STRB, the concentrations of heavy metal may increase in the oldest layers because of the mineralization of organic solids (up to 20–25%) [22]. In some cases, i.e., in the oldest layer, it may exceed the legal national standards (Danish standards, BEK. No. 1650 of 13/12/2006) for soil application when expressed as “mg/kg ds” [15].

However, as shown by Nielsen and Bruun [22], the quality of the sludge residue in general, and even for sludge residues older than 20 years, complies with the EU and Danish regulations (BEK No. 1650 of 13/12/2006), when based on the contents of heavy metals in relation to phosphorus [22].

19.5.2 Water Works Sludge

Regarding WW sludge, the overall results from full-scale STRBs and test systems showed that WW sludge is treatable in an STRB system, provided these systems are built and operated correctly and that the sludge quality from the WW are of the “normal type” (low contents of organic material, and fat).

The sludge treatment results from Hanningfield and Whitacre WWs STRBs has shown that reeds were not inhibited by WWs sludge (e.g., high ferric) as the reeds grew well and in good health in both full-scale and test system. In summary:

• The dewatering was good. Shortly after the water has drained out of the system, the sludge residue surface cracks up very well. Even after 24–25 loading days in a row, the sludge cracks up after a few days' rest.
• The area load during the tests corresponded to between 30–45 kg ds/m²/yr.
• The ferric sludge was dewatered to approximately 40% dry solids and desiccates in the trial beds.
• Filtrate water flow from the basins was good and had a curve with maximum flow of 0,02–0,03 L/m²/sec shortly after loading, which then rapidly declined during following hours.
• Iron: There was no indication of iron in the filter layers and the concentration in the reject water was low during the 5 years of test operation. This indicates that the iron stays in the sludge residue and does not clog the drainpipes.
• Due to the high iron content caused by the production process at the works using ferric sulphate (Hanningfield) for the coagulation process, it is extra important to have aerobic conditions in the sludge residue in order to avoid iron precipitation as ochre in the filter and drain. Ochre might then clog the filter.
• Vegetation: The reeds in both tests (Hanningfield and Whitacre) showed in general to have been in good health during the whole test period and the feed sludge had no negative effect on the reed growth.

In summary, the sludge treatment results obtained from Hanningfield and Whitacre WWs in full-scale and test STRB systems showed that these were able to dewater the sludge satisfactorily.