Down the Microscope - Understanding urban drainage post 2007 Hull floods

By Dr Karen Scott (@DrKarenScott)

As an environmental microbiologist I spend quite a bit of my time elbow deep in muck collecting samples to be analysed in the laboratory – which is exactly how I spent the majority of my 3.5 years PhD investigating sustainable solutions for urban drainage.

One of the streets during the Hull 2007 floods.

My research came as a result of the 2007 Hull floods, where the city was severely hit by a major flooding event which caused millions of pounds worth of damage. Blocked gully pots were partially blamed for exacerbating the flooding in the city and although it turned out there were other factors behind the severity of the floods, it did cause us to wonder what exactly was going on within the gully pots and whether the waste collected could  be managed in a more sustainable way.

Gully pots are small sumps in the roadside gutter and are important components of the urban drainage system with over 17 million in use across England and Wales, approximately 73,000 of these in Hull. Their main purpose is to collect sediment from road runoff, organic matter and litter before it enters the drainage system where potential blockages could occur. Due to the large amount of materials they collect they require regular cleaning to prevent the pots themselves becoming blocked.

A. Diagram of a large square gully pot. B. Inside the gully pot showing waste collected within.

Despite their importance, gully pot internal processes (in particular decomposition rate of their contents, which may have a significant effect upon the frequency with which the pots require emptying) have received little scientific study in this area. Due to this, it was firstly essential to examine the contents to gain a basic understanding of the processes and to establish the decomposition characteristics of the contents. Understanding the processes that occur within gully pots and determining if the environment affect these processes, is an important element in developing sustainable solutions for managing the pots.

This introductory section of my research aimed to create an initial understanding of the physical and microbial processes within the gully pot waste, the ability of it to decompose, and whether season and geographical locations affected it. To assess this, two lots of experiments were set up - in the field and in the laboratory. In the field, the city of Hull was divided into four main areas – industrial, residential, busy road and areas with high foliage. Gully pots from these four areas were randomly sampled on a monthly basis over a year (which allowed for seasonal differences to be monitored).

Modelled laboratory composting experiment

In the laboratory a composting style experiment was set up over a five week period and monitored on a more regular basis. As the decomposition processes in gully pots are unknown composting methods were used as it’s a better characterised environment and had visible similar organic waste. Setting these up in the lab made it easier to sample, control the environment (I could make sure the temperature, waste level and moisture remained constant/recorded) and ensure the samples would not be tampered with (be that people or weather etc). For all of the field experiments, samples were taken from each gully pot and taken back to the laboratory to be analysed for enzyme activity (which can indicate microbial activity, quality of organic matter and the ability of degradation), organic matter (can indicate how much of the waste can decompose) and pH. The composting trial was only analysed for organic matter to see if it was possible for the waste to decompose.

The finding from the one year field study showed that area had more of an impact than season. Differences in organic matter was observed in the seasons where it was higher in summer (potentially due to high land use e.g gardening which would decrease when the weather got worse and leaf fall during the late summer months) and lower pH in autumn and winter. These differences did not appear to affect the enzyme activity, where similar activity was observed across the seasons. Looking across the geographical area types, organic matter was considerably lower in industrial areas (due to the lack of vegetation) and pH was higher (potentially due the dumping of industrial detritus, such as cement, which was observed in samples). Enzyme activity was higher in samples with higher organic matter values, it was also present in the samples with less organic matter, but just at lower levels.  The results from the five week trial showed that the contents from the gully pots are able to decompose in modelled laboratory environments. Organic content decreased at an average rate of 0.1g of organic matter per 13g of organic matter per day. Although the rate of decomposition was observed to be slow it quantifies a previously unknown degradation process.

While significant differences in the parameters monitored between gully pots were recorded, it was difficult to show any distinct nature of the different gully pot contents. Therefore, it may be possible to treat gully waste in a homogenous manner, rather than individually, especially in a seasonal context. This may greatly assist future research to determine the activity of the contents via replica systems in a laboratory or otherwise, and can be used as a baseline when examining sustainable solutions for urban drainage waste
management.

Scott et al., 2012. An initial appraisal of waste decomposition by microbial processes within roadside gully pots. Waste Management and Research. 31(8). 
This paper can be found here.