WET News

SEPTEMBER 2014 WET NEWS 13 ADVERTISEMENT FEATURE WHY AND WHERE GRIT SHOULD BE TRAPPED? If grit is not trapped, it settles out in the sedimentation and digestion tanks, and packs around hopper-bottoms and sludge outlets. Grit removal is therefore not only desirable near the inlet to a sewage works where sedimentation tanks are used, but is essential at works where combined sewage is treated due to large quantities of grit that can be conveyed in storm flows. Undesired settlement is one issue, but just as crucial is the significant impact on mainte - nance budgets caused by early wear of mechanical equipment from highly abrasive grit. So, when cost, efficiency and TOTEX are considerations in today's decision making process, why is it that so many sewage treatment operations have allowed their grit plant to either fall into disrepair, allow grit to by-pass it, or switched it off completely? The importance of efficient grit removal at smaller works is often overlooked. If grit by-passes the grit trap on these sites it will simply be collected with the sludge in the primary sedimentation tanks. When this sludge is then W e consider inlet screens to be the front-line defence of a sewage treatment works, because by-passing rags can have an undesired impact on downstream equipment and the plant's overall efficiency. An inadequate or neglected grit plant can also lead to other inefficiencies – yet the water industry seems more tolerant - and in some cases allows for it. For example, a digester design may have up to 25% contingency capacity, just for the allowance of unwanted grit carried by imported sludge. "Why is it that so many sewage treatment operations have allowed their grit plants to fall into disrepair?" TRUE GRIT REMOVAL PROTECTING DOWNSTREAM TREATMENT Above: A Jones+Attwood Jeta ® head showing consequences of lack of grease. Top Right: Jones+Attwood Grit Classifier inactive auto lubricator. Bottom Right: A Jones+Attwood Jeta new installation. tankered off site to a larger works for processing, the grit content then goes on to create problems at the larger works. This is particularly true in digesters, which may cost up to £0.5M to dig out and clean. In our experience the most common problem with grit plants is the extreme wear to critical moving parts. This is exacerbated by the lack of adequate lubrication, which is a common symptom of neglect and poor maintenance. OPERATION AND MAINTENANCE On unmanned works, when only a weekly site visit is usually made, the time-con- trolled grit extraction process may not be in operation. This can mean that when an issue does occur, it is not spotted, so grit starts passing through the inlet works and settles with the sludge. Whilst grit problems still prevail at many sewage treatment plants, there are an increasing number of operators in the water industry who recognise the growing importance of grit removal when generating renewable energy from waste sludge. From initial health-checks through to full plant refurbish - ment and maintenance contracts, Ovivo now offers a full range of services to help plant operators maximise this potential revenue stream. Ovivo still provides original parts and services to support its heritage brands of inlet works equipment installed up to 50 years ago such as: J+A Crossflow, Detritors and other reputable EIMCO Water Technologies' systems. Our experience of grit treatment also goes back decades with certain Jones+Attwood, technology PISTA and JETA ® grit traps and Detritor ® . OVIVO also services, supplies and warrants quality replacement parts for Dorr- Oliver ® * Detritor systems. Jones+Attwood first intro- duced the grit trap to the UK in the 1950's from the Swiss company 'Pista' SA and later introduced the technology to the American-based company, Smith & Loveless. The above site examples had suffered mechanical break - down caused by lack of routine maintenance – typically insufficient greasing. "We strongly advocate the importance and benefits of regular greasing", says Neil Sandell – Ovivo Parts and Service Manager. "On face value, routine maintenance may appear expensive. However, when you take into account the impact on the grit plant, downstream equipment and labour to remove settled grit, the cost saving benefits speak for themselves". PLEASE CONTACT Neil Sandell Ovivo Parts & Service Manager +0121 511 2400 THE REAL WORLD OF MAINTENANCE PROBLEM SOLUTION OUR OPINION Installed 1993 This Jeta ® head hadn't run for several months at the time of inspection. The slew ring bearing had failed due to a lack of grease. The following items had to be replaced: • Slew ring bearing • Drive wheel • Drive pinion • Drive belt This was an unneces- sarily expensive repair – simply due to a regular amount of grease not being applied. Installed 1997 This Grit Classifier had been switched off for a few months due to the failure of the air blower unit. Upon inspection it was found that the Grit Classifier had been running prior to this without the auto lubricator in operation. The following items had to be replaced: • Bottom Bearing Assembly • Top Bearing Assembly • Auto Lubricator It is an easy check to assess whether the auto lubricator is running whilst the Grit Classifier is operating. This should be done on a regular basis. Failure to do so can result in rapid deterioration of the bottom bearing – and in this case, the top bearing. *Dorr-Oliver® is a registered trademark of FLSmidth A/S *Ovivo has no affiliation with FLSmidth A/S or its associated companies

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