Report No FR/CL0006



Jul 1997



1.1 Background

'For COD, in contrast (to BOD), the 75% removal limit was a difficult target for the majority of works. Out of the 29 works with sufficient data for the assessment only nine complied. Against the effluent limit of 125 mg/l, however, 22 works complied. Six works failed both limits.'

This conclusion led FWR in consultation with its Research Advisory Group to formulate a brief for a follow-up study to investigate in more detail the extent and significance of the problem of COD removal. Invitations to tender for this research work on "Chemical Oxygen Demand of Sewage Effluents" were issued in October 1995 and the project formally started on 1 March 1996.

1.2 Objectives

'To investigate the nature of COD in sewage effluents (containing minimal trade effluent) which appears not to be removed in conventional treatment processes: to consider the environmental implications of COD components in effluents discharged to rivers and potential impacts on water treatment plants and drinking water supplies.'

1.3 Organisational arrangements and responsibilities

Foundation for Water Research (FWR): The FWR was responsible for the management of the overall programme through the appointment of a programme manager whose role was to liaise with all the participants and undertake report production and dissemination.

Montgomery Watson Ltd: The review of published information, collection, collation and interpretation of data and preparation of the project report was undertaken by the research contractor Montgomerey Watson Ltd.

Regulators: The environmental regulators comprising the Environment Agency, and the Scottish Environmental Protection Agency and the Department of the Environment for Northern Ireland represented by SNIFFER (the Scotland and Northern Ireland Forum for Environmental Research) funded the programme management by FWR and the work of the specialist sub-contractor Montgomerey Watson Ltd.

Operators: The operators (comprising the Water Service Companies of England and Wales) together with the sewage treatment operators in Scotland and Northern Ireland. permitted the data collected for the initial NCTP project to be used for this study and provided additional data where these were available.

Steering Group: A Steering group representing all participants in the project was formed to oversee the study and approve outputs. A representative of OFWAT was also invited to join the Steering Group.

1.4 Findings

1.4.1 Influent and effluent quality

Influent COD levels are well correlated with influent BOD and SS. Average COD:BOD ratios in influents generally lie in the range 2-4.5.

COD concentrations in sewage effluents vary widely but average values generally lie within the range 10-100 mg/l. The correlations between COD, BOD and SS are less strong in effluents than in influents but are still evident. Average COD:BOD ratios in effluents vary widely between works and may lie between about 5 and 20.

1.4.2 Compliance with UWWTD standards

When the limits are applied to each sample on an "either/or" basis as proposed in the draft DoE Guidance Note, the total number of failures reduces to 6 (STWs 13, 31, 64, 69, 70 and 78). Of the 29 works assessed for COD removal, 3 also fail one or other of the UWWTD standards for BOD but there are no failures of the BOD limits on an "either/or" basis. Two of the works that fail a BOD limit also fail both COD limits.

Analysis of the works failing either COD limit shows no clear pattern in terms of the works characteristics provided in the NCTP data (type of works, size of works, trade effluent %. BOD consent, SS consent, AmmN consent).

Analysis of the works failing both COD limits does show that none have consents for ammonia and that 3 out of 6 have relatively relaxed BOD:SS standards of 40:40 or more.

1.4.3 Treatabilitv

There does not appear to be a rigorous definition of "hard COD". It is often used to describe the total residual of COD present in an effluent and as such would include both the inert non-treatable fraction and the fraction of more difficult to treat material not treated at the particular works. Using this definition, embracing both non-treated and non-treatable components of COD, the NCTP data indicate a typical level of hard COD for a well-performing STW of 40-70 mg/l. The absolute minimum value, representing the nontreatable fraction, appears to be about 10-20 mg/l.

The treatability of the COD in an influent depends on both the character of the influent and of the works treating it. No single model has been found that gives a good explanation of treatability. However, from the literature and the data analysis, the presence of one or more of the following factors increases the probability of poor treatability (% removal of COD):

These factors seem to be less common in large works than in small or medium sized works.

Characteristics of sewage treatment processes that are reported to favour COD removal are:

A two stage process comprising a high rate process (high sludge loading rate) followed by extended aeration (low sludge loading rate, high sludge age) has been reported to be particularly effective at COD removal and should be capable of producing an ef fluent with mean COD concentration around 40 mg/l.

1.4.4 Nature and consequences of COD in effluents

Without a better knowledge of the constituents of hard COD, the environmental significance is difficult to assess.

1.5 Recommendations

  1. More detailed analysis is needed at individual STWs to understand the factors that affect COD removal and its variation. A simple model or indicator (such as COD:BOD ratio or denitrification) is needed to assess the number of works that may be at risk from COD failure on a national scale.
  2. It is likely that the parameters and possibly the form of any predictive model (e.g. regression equation) will vary between different types of treatment plant. Daily composite samples are needed to allow the correlation of effluent quality with lagged influent quality to be investigated. Since the influent COD:BOD ratio is one of the suggested indicators of performance, it would be informative if the works selected for detailed analysis include variable influent COD:BOD ratios.
  3. The six STWs that would fail the UWWTD standards should be studied to assess what scope there is for making process changes to ensure their compliance in terms of COD.
  4. Laboratory scale or pilot plant work under controlled conditions would be of assistance in defining more closely how the characteristics of the most common processes (e.g. activated sludge, percolating filters) affect COD removal.
  5. More detailed analysis of the constituents of COD in sewage effluents is needed to assess whether COD levels in effluents are of environmental significance.
  6. Using the data derived in 2, and 3, in conjunction with an indicator or model of the type recommended in 1 and 2, an estimate could be made of the likely cost of complying with the COD provisions of the UWWTD.

Copies of the report are available from FWR, price 10.00, less 20% to FWR Members.