Report No FR/WR0003



APRIL 1994



Project P006 proposes a jointly funded research programme with MAFF, DoE and FWR to study, by means of a series of field trials, the long-term effects of sewage sludge on agricultural soils and in particular heavy metal impacts on soil fertility. With the ending of the five year agreement with the FWR and the water undertakings, the water industry funding to this project will now be through the Operators Collaborative Programme administered by UK Water Industry Research Ltd. It is anticipated that the project will commence in spring 1994.

The design of the trials will require sludges with concentrations of Zn, Cu and Cd higher than average. All plot applications will have the same organic matter additions by appropriate applications of sludge of low metal concentration. In assessing the feasibility of this design, data from the DoE 1990/91 sludge survey were screened for suitable sludges. Whilst there appeared to be many candidate sludges for the trial, on checking many were found to occur in too smaller quantities, or their metal concentrations were now thought to be significantly lower than reported in the 1990/91 database, or they were no longer available or accessible.

This preparatory study was, therefore, initiated and funded by FWR to ensure that this important series of field trials could be established with the appropriate levels of metal addition from existing sewage sludges. It was necessary to identify and verify the existence and amounts of sludges with suitable heavy metal profiles. Contacts with the water undertakings were approached to select the most suitable sludges for further evaluation by sampling and analysis to be undertaken by WRc.


All of the ten water service companies were contacted in England and Wales, and also the most promising Scottish regional councils; Borders, Dumfries and Galloway, Highland and Strathclyde. Written replies were received from Borders, Southern Water, Severn-Trent and Yorkshire Water containing more up to date or detailed analyses of sludges which could be suitable.

From past data and further discussion with the water companies it soon became clear that to meet the requirements of the proposed trial, it was important to identify a high level zinc sludge first. This was because high level zinc sludges were less common than past data had indicated and the zinc concentration in this sludge and its organic matter level would indicate the most suitable levels of copper and cadmium in the other two sludges required. In order to calculate the quantity of sludge needed at a specified metal quality, certain assumptions about the field site soils, the blank sludge and the high metal sludge dry solids were necessary. These are summarised in Table 1 . The proposed zinc additions for the main treatments in the field trial based on these assumptions are shown in Table 2. All estimates in Table 2 are in fact calculated directly from the assumptions on a spreadsheet. This allows sludge of different metal level, organic matter and dry solids to be quickly assessed using the same formulae. From Table 2 it can be seen that if a sludge cake was available at 50 percent organic matter and a zinc concentration of 5000 mg/kg dry solids, approximately the highest likely to be available, the organic matter additions would be approximately 3.7 percent above background for each treatment. Also assuming similar organic matter content for all sludges copper and cadmium concentrations in the other two high metal sludges would then need to be 3571 mg/kg and 71 mg/kg respectively.


Following further discussion the most promising sludge with a high level of zinc but a low level of copper was identified. This was in the midlands at Walsall Wood and the large stockpile was sampled from 18 locations (Table 3). The sludge cake was originally reported as having in 1991 , 3200, 80 and 4 mg/kg of Zn, Cu and Cd respectively. However as analysis shows in Table 3 the highest area of the stockpile contained only 1780 mg/kg Zn and Cu was significantly higher than originally reported at 842 mg/kg. No locations were found with exceptionally high Zn as required and on average the metal levels were two and a half times that of a domestic sludge, about a fifth of the levels hoped for in the proposed trial. Also the wide range of locations sampled in the large stockpile, approximately 100 metres long, 20 metres wide and up to 8 metres high, indicated little variation in quality. The age of the sludge also varied widely at the locations sampled from many years to less than one year. Dry solids, however, did reflect the approximate age of the samples taken. Three recent locations less than 1 year old had an average dry solids of 30 percent, but the remaining older samples averaged 74 percent dry solids. A difference in percent loss on ignition was also found between these two groups. The recent samples contained 40% combustible material and three older samples only 20%.

These results for Walsall Wood sludge cake turned out to vary widely from the original requirements and entering it's quality data into the quantity formulae built into Table 2 indicated it would be unsuitable for the proposed trial (Table 4). With a Zn concentration of 1234 mg/kg in the sludge and estimating the organic matter content as 21% from the loss on ignition data, up to 900 tds/ha would have to be applied to achieve the highest target Zn soil concentration over four years. The total wet volume of high Zn sludge required for all six sites would be 255 tonnes. Amounts of blank sludge required for all treatments would total 1233 wet tonnes, assuming the blank had more usual dry matter and organic matter of 25% and 50% respectively. Also blank additions would be so high that metal added in the blank would significantly increase soil concentration at the lowest treatment numbers 2 and 3. The large amount of dry solids added would elevate soil background organic matter by 15% possibly leading to a total soil organic matter level of 17%.

If this sludge were acceptable for the trial then the high Cu and Cd sludges required for the other treatments should have 881 and 18 mg/kg Cu and Cd respectively. Also these other two sludges would both be required to have a loss of ignition value of 21% similar to Walsall Wood. In addition the Walsall Wood sludge had a fairly high concentration of Cu (706 mg/kg), therefore, copper in the soil would be elevated in the Zn treatments to a similar level as the Cu treatments.

The most promising high level cadmium sludge was reported to be at Aldwarke treatment works in Yorkshire. This was followed up with a sampling exercise similar to Walsall Wood to survey the whole site. On arrival at Aldwarke it transpired that the sludge had been levelled across a wide area of several hectares and was in fact a very heavily sludged field. Despite this samples were taken for analysis and are given in Table 5. Unfortunately Cd levels were on average only 4.6 mg/kg indicating this was now no more than heavily sludged soil.

A high copper sludge was identified in Borders Regional Council at Selkirk, however, further inquiries suggested the production may be too low and no stockpile was present. When sludge had been stored for too long on site in the past, complaints had been received about the smell and so it would not be possible to stockpile in the future. Another high copper sludge cake was originally identified in Southern Water at Aylesford. Further inquiries into this source revealed the quality with respect to copper had dramatically improved over the last three years and this was illustrated in a graph supplied by Southern Water (Figure 1).


Sludge cake significantly high in any of the three target metals while low in the remaining two have not been found except for the high Cu undigested sludge cake at Selkirk in Scotland. The best alternatives have been identified and are primarily a sludge cake high in Zn, 'medium' in Cu but low in Cd, and a sludge cake high in Cd but relatively low in Zn and Cu. A digested sludge cake closest to fulfilling the high Zn requirements has been identified in the Severn Trent area (Minworth) and the best high Cd cake sludge identified in the Thames area (Perry Oaks). The availability of Minworth and Perry Oaks sludges should be very good as large quantities are stored on site. The availability of Selkirk sludge will only be on a daily production basis, however, it is thought that arrangements should be flexible enough to allow a lorry load at a time to be collected as required. A lorry load has been estimated to be two days production.

Reliable data on metal concentrations, dry solids and organic matter have been obtained for all three sludges. However, further results are being obtained for all three sludges to confirm their quality. Perry Oaks site has been visited recently and sludge samples taken for analysis. Results are shown in Table 6. Seven likely sites were sampled and the sample from the most promising site (7) judged by appearance, was divided into four before analysis.

Thames Water have also been contacted with regard to suitable blank sludges and preliminary results have allowed two, one digested and one raw, to be identified. The quantities of each sludge required to meet the experimental design have been calculated according to the latest results available (Tables 7-9). The dry solids and organic matter additions to the field plots are indicated in the tables and estimated rise in soil organic matter.

The primary contacts who allowed identification of the three high metal sludge cakes are listed in Table 10.

A sample of Selkirk sludge has been taken recently and analysis for dry solids is being carried out by Borders Regional Council and for organic matter and metals by WRc.

A visit is planned to Minworth to confirm data provided by STW, to survey the site of sludge storage and sample cake for dry solids and organic matter.


It is proposed that the best available sludges identified be used for the proposed field trials:

  1. High zinc treatments; Minworth digested sludge cake - Severn Trent Water
  2. High copper treatments; Selkirk undigested sludge cake - Borders Regional Council
  3. High cadmium treatments; Perry Oaks air-dried sludge cake - Thames Water

Copies of the report are available from FWR, price 15.00, less 20% to FWR Members.